From c8dc5992d13d05e77d8c1eab13ce7795f5bf9454 Mon Sep 17 00:00:00 2001
From: MarkBaker <mark@lange.demon.co.uk>
Date: Sat, 27 Mar 2021 21:11:18 +0100
Subject: [PATCH 1/8] Implementation of the CHITEST() statistical function

---
 CHANGELOG.md                                  |  1 +
 .../Calculation/Calculation.php               |  4 +-
 .../Statistical/Distributions/ChiSquared.php  | 41 +++++++++++++++++++
 .../Functions/Statistical/ChiTestTest.php     | 25 +++++++++++
 .../data/Calculation/Statistical/CHITEST.php  | 29 +++++++++++++
 5 files changed, 98 insertions(+), 2 deletions(-)
 create mode 100644 tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiTestTest.php
 create mode 100644 tests/data/Calculation/Statistical/CHITEST.php

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9b46323f52..d6d252d533 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 
 ### Added
 
+- Implemented the CHITEST() Statistical function.
 - Support for ActiveSheet and SelectedCells in the ODS Reader and Writer. [PR #1908](https://github.com/PHPOffice/PhpSpreadsheet/pull/1908)
 
 ### Changed
diff --git a/src/PhpSpreadsheet/Calculation/Calculation.php b/src/PhpSpreadsheet/Calculation/Calculation.php
index 6e98fcb083..62524c37a0 100644
--- a/src/PhpSpreadsheet/Calculation/Calculation.php
+++ b/src/PhpSpreadsheet/Calculation/Calculation.php
@@ -518,12 +518,12 @@ class Calculation
         ],
         'CHITEST' => [
             'category' => Category::CATEGORY_STATISTICAL,
-            'functionCall' => [Functions::class, 'DUMMY'],
+            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'test'],
             'argumentCount' => '2',
         ],
         'CHISQ.TEST' => [
             'category' => Category::CATEGORY_STATISTICAL,
-            'functionCall' => [Functions::class, 'DUMMY'],
+            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'test'],
             'argumentCount' => '2',
         ],
         'CHOOSE' => [
diff --git a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php
index 636189b52b..766a23d097 100644
--- a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php
+++ b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php
@@ -82,4 +82,45 @@ public static function inverse($probability, $degrees)
 
         return $newtonRaphson->execute($probability);
     }
+
+    public static function test($actual, $expected)
+    {
+        $rows = count($actual);
+        $actual = Functions::flattenArray($actual);
+        $expected = Functions::flattenArray($expected);
+        $columns = count($actual) / $rows;
+
+        $countActuals = count($actual);
+        $countExpected = count($expected);
+        if ($countActuals !== $countExpected || $countActuals === 1) {
+            return Functions::NAN();
+        }
+
+        $result = 0.0;
+        for ($i = 0; $i < $countActuals; ++$i) {
+            if ($expected[$i] == 0.0) {
+                return Functions::DIV0();
+            } elseif($expected[$i] < 0.0) {
+                return Functions::NAN();
+            }
+            $result += (($actual[$i] - $expected[$i]) ** 2) / $expected[$i];
+        }
+
+        $degrees = self::degrees($rows, $columns);
+
+        $result = self::distribution($result, $degrees);
+
+        return $result;
+    }
+
+    protected static function degrees(int $rows, int $columns): int
+    {
+        if ($rows === 1) {
+            return $columns - 1;
+        } elseif ($columns === 1) {
+            return $rows - 1;
+        }
+
+        return ($columns - 1) * ($rows - 1);
+    }
 }
diff --git a/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiTestTest.php b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiTestTest.php
new file mode 100644
index 0000000000..95f252cd49
--- /dev/null
+++ b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiTestTest.php
@@ -0,0 +1,25 @@
+<?php
+
+namespace PhpOffice\PhpSpreadsheetTests\Calculation\Functions\Statistical;
+
+use PhpOffice\PhpSpreadsheet\Calculation\Statistical;
+use PHPUnit\Framework\TestCase;
+
+class ChiTestTest extends TestCase
+{
+    /**
+     * @dataProvider providerCHITEST
+     *
+     * @param mixed $expectedResult
+     */
+    public function testCHITEST($expectedResult, $actual, $expected): void
+    {
+        $result = Statistical\Distributions\ChiSquared::test($actual, $expected);
+        self::assertEqualsWithDelta($expectedResult, $result, 1E-12);
+    }
+
+    public function providerCHITEST()
+    {
+        return require 'tests/data/Calculation/Statistical/CHITEST.php';
+    }
+}
diff --git a/tests/data/Calculation/Statistical/CHITEST.php b/tests/data/Calculation/Statistical/CHITEST.php
new file mode 100644
index 0000000000..aab9ad51e6
--- /dev/null
+++ b/tests/data/Calculation/Statistical/CHITEST.php
@@ -0,0 +1,29 @@
+<?php
+
+return [
+    [
+        0.0152703571482,
+        [[112, 76], [85, 95]],
+        [[100, 85], [70, 90]],
+    ],
+    [
+        0.000308192027,
+        [[58, 11], [10, 35], [25, 23]],
+        [[45.35, 17.56], [16.09, 47.65], [18.44, 16.91]],
+    ],
+    [
+        '#NUM!',
+        [[58, 11], [10, 35], [25]],
+        [[45.35, 17.56], [16.09, 47.65], [18.44, 16.91]],
+    ],
+    [
+        '#DIV/0!',
+        [[112, 76], [85, 95]],
+        [[100, 0], [70, 90]],
+    ],
+    [
+        '#NUM!',
+        [[112, 76], [85, 95]],
+        [[100, 85], [-70, 90]],
+    ],
+];

From 8f484064bf0822a570957e453b82531becfe3101 Mon Sep 17 00:00:00 2001
From: MarkBaker <mark@lange.demon.co.uk>
Date: Sat, 27 Mar 2021 21:34:40 +0100
Subject: [PATCH 2/8] A couple of additional edge case tests (rows = 1, columns
 = 1)

---
 .../Statistical/Distributions/ChiSquared.php       |  2 +-
 .../Functions/Statistical/ChiTestTest.php          |  2 ++
 tests/data/Calculation/Statistical/CHITEST.php     | 14 ++++++++++++--
 3 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php
index 766a23d097..dfd090de6e 100644
--- a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php
+++ b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php
@@ -100,7 +100,7 @@ public static function test($actual, $expected)
         for ($i = 0; $i < $countActuals; ++$i) {
             if ($expected[$i] == 0.0) {
                 return Functions::DIV0();
-            } elseif($expected[$i] < 0.0) {
+            } elseif ($expected[$i] < 0.0) {
                 return Functions::NAN();
             }
             $result += (($actual[$i] - $expected[$i]) ** 2) / $expected[$i];
diff --git a/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiTestTest.php b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiTestTest.php
index 95f252cd49..5f9f361ff4 100644
--- a/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiTestTest.php
+++ b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiTestTest.php
@@ -11,6 +11,8 @@ class ChiTestTest extends TestCase
      * @dataProvider providerCHITEST
      *
      * @param mixed $expectedResult
+     * @param mixed $actual
+     * @param mixed $expected
      */
     public function testCHITEST($expectedResult, $actual, $expected): void
     {
diff --git a/tests/data/Calculation/Statistical/CHITEST.php b/tests/data/Calculation/Statistical/CHITEST.php
index aab9ad51e6..6f5cd05c6f 100644
--- a/tests/data/Calculation/Statistical/CHITEST.php
+++ b/tests/data/Calculation/Statistical/CHITEST.php
@@ -8,8 +8,18 @@
     ],
     [
         0.000308192027,
-        [[58, 11], [10, 35], [25, 23]],
-        [[45.35, 17.56], [16.09, 47.65], [18.44, 16.91]],
+        [[58, 35], [11, 25], [10, 23]],
+        [[45.35, 47.65], [17.56, 18.44], [16.09, 16.91]],
+    ],
+    [
+        0.015888560447,
+        [[58], [11], [10]],
+        [[45.35], [17.56], [16.09]],
+    ],
+    [
+        0.008682970191,
+        [[58, 35]],
+        [[45.35, 47.65]],
     ],
     [
         '#NUM!',

From 6e1f93e2a13cfb617f75077ef1ceef894f639d28 Mon Sep 17 00:00:00 2001
From: MarkBaker <mark@lange.demon.co.uk>
Date: Sat, 27 Mar 2021 23:06:10 +0100
Subject: [PATCH 3/8] Implement Left-tailed distribution of ChiSquared, for
 both cdf and pdf

---
 .../Calculation/Calculation.php               |  2 +-
 .../Statistical/Distributions/ChiSquared.php  | 44 +++++++++++++++++++
 .../Functions/Statistical/ChiDist2Test.php    | 31 +++++++++++++
 .../data/Calculation/Statistical/CHIDIST2.php | 40 +++++++++++++++++
 4 files changed, 116 insertions(+), 1 deletion(-)
 create mode 100644 tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDist2Test.php
 create mode 100644 tests/data/Calculation/Statistical/CHIDIST2.php

diff --git a/src/PhpSpreadsheet/Calculation/Calculation.php b/src/PhpSpreadsheet/Calculation/Calculation.php
index 62524c37a0..fe582bf7b6 100644
--- a/src/PhpSpreadsheet/Calculation/Calculation.php
+++ b/src/PhpSpreadsheet/Calculation/Calculation.php
@@ -493,7 +493,7 @@ class Calculation
         ],
         'CHISQ.DIST' => [
             'category' => Category::CATEGORY_STATISTICAL,
-            'functionCall' => [Functions::class, 'DUMMY'],
+            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'distributionLeftTail'],
             'argumentCount' => '3',
         ],
         'CHISQ.DIST.RT' => [
diff --git a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php
index dfd090de6e..3a149f2b9c 100644
--- a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php
+++ b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php
@@ -47,6 +47,50 @@ public static function distribution($value, $degrees)
         return 1 - (Gamma::incompleteGamma($degrees / 2, $value / 2) / Gamma::gammaValue($degrees / 2));
     }
 
+    /**
+     * CHIDIST.
+     *
+     * Returns the one-tailed probability of the chi-squared distribution.
+     *
+     * @param mixed (float) $value Value for the function
+     * @param mixed (int) $degrees degrees of freedom
+     * @param mixed $cumulative
+     *
+     * @return float|string
+     */
+    public static function distributionLeftTail($value, $degrees, $cumulative)
+    {
+        $value = Functions::flattenSingleValue($value);
+        $degrees = Functions::flattenSingleValue($degrees);
+        $cumulative = Functions::flattenSingleValue($cumulative);
+
+        try {
+            $value = self::validateFloat($value);
+            $degrees = self::validateInt($degrees);
+            $cumulative = self::validateBool($cumulative);
+        } catch (Exception $e) {
+            return $e->getMessage();
+        }
+
+        if ($degrees < 1) {
+            return Functions::NAN();
+        }
+        if ($value < 0) {
+            if (Functions::getCompatibilityMode() == Functions::COMPATIBILITY_GNUMERIC) {
+                return 1;
+            }
+
+            return Functions::NAN();
+        }
+
+        if ($cumulative === true) {
+            return 1 - self::distribution($value, $degrees);
+        }
+
+        return (($value ** (($degrees / 2) - 1) * exp(-$value / 2))) /
+            ((2 ** ($degrees / 2)) * Gamma::gammaValue($degrees / 2));
+    }
+
     /**
      * CHIINV.
      *
diff --git a/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDist2Test.php b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDist2Test.php
new file mode 100644
index 0000000000..ce1cd4e5ca
--- /dev/null
+++ b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDist2Test.php
@@ -0,0 +1,31 @@
+<?php
+
+namespace PhpOffice\PhpSpreadsheetTests\Calculation\Functions\Statistical;
+
+use PhpOffice\PhpSpreadsheet\Calculation\Functions;
+use PhpOffice\PhpSpreadsheet\Calculation\Statistical;
+use PHPUnit\Framework\TestCase;
+
+class ChiDist2Test extends TestCase
+{
+    protected function setUp(): void
+    {
+        Functions::setCompatibilityMode(Functions::COMPATIBILITY_EXCEL);
+    }
+
+    /**
+     * @dataProvider providerCHIDIST
+     *
+     * @param mixed $expectedResult
+     */
+    public function testCHIDIST($expectedResult, ...$args): void
+    {
+        $result = Statistical\Distributions\ChiSquared::distributionLeftTail(...$args);
+        self::assertEqualsWithDelta($expectedResult, $result, 1E-12);
+    }
+
+    public function providerCHIDIST()
+    {
+        return require 'tests/data/Calculation/Statistical/CHIDIST2.php';
+    }
+}
diff --git a/tests/data/Calculation/Statistical/CHIDIST2.php b/tests/data/Calculation/Statistical/CHIDIST2.php
new file mode 100644
index 0000000000..5bc6761732
--- /dev/null
+++ b/tests/data/Calculation/Statistical/CHIDIST2.php
@@ -0,0 +1,40 @@
+<?php
+
+return [
+    [
+        0.520499877813,
+        0.5, 1, true,
+    ],
+    [
+        0.207553748710,
+        2, 3, false,
+    ],
+    [
+        0.111565080074,
+        3, 2, false,
+    ],
+    [
+        0.776869839852,
+        3, 2, true,
+    ],
+    [
+        '#VALUE!',
+        'NaN', 3, true,
+    ],
+    [
+        '#VALUE!',
+        2, 'NaN', true,
+    ],
+    [
+        '#VALUE!',
+        2, 3, 'NaN',
+    ],
+    'Value < 0' => [
+        '#NUM!',
+        -8, 3, true,
+    ],
+    'Degrees < 1' => [
+        '#NUM!',
+        8, 0, true,
+    ],
+];

From a1fd528bc66d127d1a7e15a78402eaeacbaaacc6 Mon Sep 17 00:00:00 2001
From: MarkBaker <mark@lange.demon.co.uk>
Date: Sat, 27 Mar 2021 23:10:00 +0100
Subject: [PATCH 4/8] Update change log

---
 CHANGELOG.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index d6d252d533..d7b123efe7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,7 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 
 ### Added
 
-- Implemented the CHITEST() Statistical function.
+- Implemented the CHITEST() and CHISQ.DIST() Statistical function.
 - Support for ActiveSheet and SelectedCells in the ODS Reader and Writer. [PR #1908](https://github.com/PHPOffice/PhpSpreadsheet/pull/1908)
 
 ### Changed

From ced7554d1b55c26b7a5aede3dcc4286ed94340ad Mon Sep 17 00:00:00 2001
From: MarkBaker <mark@lange.demon.co.uk>
Date: Sun, 28 Mar 2021 15:22:26 +0200
Subject: [PATCH 5/8] Update change log

---
 .../Calculation/Calculation.php               |  8 ++---
 .../Calculation/Statistical.php               | 14 ++++----
 .../Statistical/Distributions/ChiSquared.php  | 10 +++---
 .../Statistical/Distributions/GammaBase.php   |  3 +-
 .../Distributions/NewtonRaphson.php           | 33 +++++++++++++++++--
 ...iDist2Test.php => ChiDistLeftTailTest.php} |  4 +--
 ...iDistTest.php => ChiDistRightTailTest.php} |  4 +--
 ...ChiInvTest.php => ChiInvRightTailTest.php} | 12 ++++---
 .../{CHIDIST2.php => CHIDISTLeftTail.php}     | 24 ++++++++++++++
 .../{CHIDIST.php => CHIDISTRightTail.php}     |  0
 .../{CHIINV.php => CHIINVRightTail.php}       | 12 +++++--
 11 files changed, 94 insertions(+), 30 deletions(-)
 rename tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/{ChiDist2Test.php => ChiDistLeftTailTest.php} (92%)
 rename tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/{ChiDistTest.php => ChiDistRightTailTest.php} (92%)
 rename tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/{ChiInvTest.php => ChiInvRightTailTest.php} (59%)
 rename tests/data/Calculation/Statistical/{CHIDIST2.php => CHIDISTLeftTail.php} (61%)
 rename tests/data/Calculation/Statistical/{CHIDIST.php => CHIDISTRightTail.php} (100%)
 rename tests/data/Calculation/Statistical/{CHIINV.php => CHIINVRightTail.php} (82%)

diff --git a/src/PhpSpreadsheet/Calculation/Calculation.php b/src/PhpSpreadsheet/Calculation/Calculation.php
index fe582bf7b6..c4a88bc3e5 100644
--- a/src/PhpSpreadsheet/Calculation/Calculation.php
+++ b/src/PhpSpreadsheet/Calculation/Calculation.php
@@ -488,7 +488,7 @@ class Calculation
         ],
         'CHIDIST' => [
             'category' => Category::CATEGORY_STATISTICAL,
-            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'distribution'],
+            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'distributionRightTail'],
             'argumentCount' => '2',
         ],
         'CHISQ.DIST' => [
@@ -498,12 +498,12 @@ class Calculation
         ],
         'CHISQ.DIST.RT' => [
             'category' => Category::CATEGORY_STATISTICAL,
-            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'distribution'],
+            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'distributionRightTail'],
             'argumentCount' => '2',
         ],
         'CHIINV' => [
             'category' => Category::CATEGORY_STATISTICAL,
-            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'inverse'],
+            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'inverseRightTail'],
             'argumentCount' => '2',
         ],
         'CHISQ.INV' => [
@@ -513,7 +513,7 @@ class Calculation
         ],
         'CHISQ.INV.RT' => [
             'category' => Category::CATEGORY_STATISTICAL,
-            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'inverse'],
+            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'inverseRightTail'],
             'argumentCount' => '2',
         ],
         'CHITEST' => [
diff --git a/src/PhpSpreadsheet/Calculation/Statistical.php b/src/PhpSpreadsheet/Calculation/Statistical.php
index 5f55743115..01caba14a6 100644
--- a/src/PhpSpreadsheet/Calculation/Statistical.php
+++ b/src/PhpSpreadsheet/Calculation/Statistical.php
@@ -297,8 +297,8 @@ public static function BINOMDIST($value, $trials, $probability, $cumulative)
      *
      * @Deprecated 1.18.0
      *
-     * @see Statistical\Distributions\ChiSquared::distribution()
-     *      Use the distribution() method in the Statistical\Distributions\ChiSquared class instead
+     * @see Statistical\Distributions\ChiSquared::distributionRightTail()
+     *      Use the distributionRightTail() method in the Statistical\Distributions\ChiSquared class instead
      *
      * @param float $value Value for the function
      * @param float $degrees degrees of freedom
@@ -307,7 +307,7 @@ public static function BINOMDIST($value, $trials, $probability, $cumulative)
      */
     public static function CHIDIST($value, $degrees)
     {
-        return Statistical\Distributions\ChiSquared::distribution($value, $degrees);
+        return Statistical\Distributions\ChiSquared::distributionRightTail($value, $degrees);
     }
 
     /**
@@ -317,8 +317,8 @@ public static function CHIDIST($value, $degrees)
      *
      * @Deprecated 1.18.0
      *
-     * @see Statistical\Distributions\ChiSquared::inverse()
-     *      Use the inverse() method in the Statistical\Distributions\ChiSquared class instead
+     * @see Statistical\Distributions\ChiSquared::inverseRightTail()
+     *      Use the inverseRightTail() method in the Statistical\Distributions\ChiSquared class instead
      *
      * @param float $probability Probability for the function
      * @param float $degrees degrees of freedom
@@ -327,7 +327,7 @@ public static function CHIDIST($value, $degrees)
      */
     public static function CHIINV($probability, $degrees)
     {
-        return Statistical\Distributions\ChiSquared::inverse($probability, $degrees);
+        return Statistical\Distributions\ChiSquared::inverseRightTail($probability, $degrees);
     }
 
     /**
@@ -2159,7 +2159,7 @@ public static function TDIST($value, $degrees, $tails)
     /**
      * TINV.
      *
-     * Returns the one-tailed probability of the chi-squared distribution.
+     * Returns the one-tailed probability of the Student-T distribution.
      *
      * @Deprecated 1.18.0
      *
diff --git a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php
index 3a149f2b9c..409e5883a2 100644
--- a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php
+++ b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php
@@ -21,7 +21,7 @@ class ChiSquared
      *
      * @return float|string
      */
-    public static function distribution($value, $degrees)
+    public static function distributionRightTail($value, $degrees)
     {
         $value = Functions::flattenSingleValue($value);
         $degrees = Functions::flattenSingleValue($degrees);
@@ -84,7 +84,7 @@ public static function distributionLeftTail($value, $degrees, $cumulative)
         }
 
         if ($cumulative === true) {
-            return 1 - self::distribution($value, $degrees);
+            return 1 - self::distributionRightTail($value, $degrees);
         }
 
         return (($value ** (($degrees / 2) - 1) * exp(-$value / 2))) /
@@ -94,14 +94,14 @@ public static function distributionLeftTail($value, $degrees, $cumulative)
     /**
      * CHIINV.
      *
-     * Returns the one-tailed probability of the chi-squared distribution.
+     * Returns the inverse of the right-tailed probability of the chi-squared distribution.
      *
      * @param mixed (float) $probability Probability for the function
      * @param mixed (int) $degrees degrees of freedom
      *
      * @return float|string
      */
-    public static function inverse($probability, $degrees)
+    public static function inverseRightTail($probability, $degrees)
     {
         $probability = Functions::flattenSingleValue($probability);
         $degrees = Functions::flattenSingleValue($degrees);
@@ -152,7 +152,7 @@ public static function test($actual, $expected)
 
         $degrees = self::degrees($rows, $columns);
 
-        $result = self::distribution($result, $degrees);
+        $result = self::distributionRightTail($result, $degrees);
 
         return $result;
     }
diff --git a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/GammaBase.php b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/GammaBase.php
index 3f76787d1e..89170f7cad 100644
--- a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/GammaBase.php
+++ b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/GammaBase.php
@@ -30,14 +30,15 @@ protected static function calculateInverse(float $probability, float $alpha, flo
         $xLo = 0;
         $xHi = $alpha * $beta * 5;
 
-        $x = $xNew = 1;
         $dx = 1024;
+        $x = $xNew = 1;
         $i = 0;
 
         while ((abs($dx) > Functions::PRECISION) && (++$i <= self::MAX_ITERATIONS)) {
             // Apply Newton-Raphson step
             $result = self::calculateDistribution($x, $alpha, $beta, true);
             $error = $result - $probability;
+
             if ($error == 0.0) {
                 $dx = 0;
             } elseif ($error < 0.0) {
diff --git a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php
index 298cdfaf34..9a0ba4ed27 100644
--- a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php
+++ b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php
@@ -10,18 +10,45 @@ class NewtonRaphson
 
     protected $callback;
 
+    protected $low = 100.0;
+
+    protected $high = 0.0;
+
+    protected $dx = 1;
+
     public function __construct(callable $callback)
     {
         $this->callback = $callback;
     }
 
+    public function setLow(float $lowValue): self
+    {
+        $this->low = $lowValue;
+
+        return $this;
+    }
+
+    public function setHigh(float $highValue): self
+    {
+        $this->low = $highValue;
+
+        return $this;
+    }
+
+    public function setDx(float $dxValue): self
+    {
+        $this->dx = $dxValue;
+
+        return $this;
+    }
+
     public function execute($probability)
     {
-        $xLo = 100;
-        $xHi = 0;
+        $xLo = $this->low;
+        $xHi = $this->high;
 
+        $dx = $this->dx;
         $x = $xNew = 1;
-        $dx = 1;
         $i = 0;
 
         while ((abs($dx) > Functions::PRECISION) && ($i++ < self::MAX_ITERATIONS)) {
diff --git a/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDist2Test.php b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDistLeftTailTest.php
similarity index 92%
rename from tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDist2Test.php
rename to tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDistLeftTailTest.php
index ce1cd4e5ca..3c7a8d4ec6 100644
--- a/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDist2Test.php
+++ b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDistLeftTailTest.php
@@ -6,7 +6,7 @@
 use PhpOffice\PhpSpreadsheet\Calculation\Statistical;
 use PHPUnit\Framework\TestCase;
 
-class ChiDist2Test extends TestCase
+class ChiDistLeftTailTest extends TestCase
 {
     protected function setUp(): void
     {
@@ -26,6 +26,6 @@ public function testCHIDIST($expectedResult, ...$args): void
 
     public function providerCHIDIST()
     {
-        return require 'tests/data/Calculation/Statistical/CHIDIST2.php';
+        return require 'tests/data/Calculation/Statistical/CHIDISTLeftTail.php';
     }
 }
diff --git a/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDistTest.php b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDistRightTailTest.php
similarity index 92%
rename from tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDistTest.php
rename to tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDistRightTailTest.php
index 9dc7326c6f..26bf5ab764 100644
--- a/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDistTest.php
+++ b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDistRightTailTest.php
@@ -6,7 +6,7 @@
 use PhpOffice\PhpSpreadsheet\Calculation\Statistical;
 use PHPUnit\Framework\TestCase;
 
-class ChiDistTest extends TestCase
+class ChiDistRightTailTest extends TestCase
 {
     protected function setUp(): void
     {
@@ -26,6 +26,6 @@ public function testCHIDIST($expectedResult, ...$args): void
 
     public function providerCHIDIST()
     {
-        return require 'tests/data/Calculation/Statistical/CHIDIST.php';
+        return require 'tests/data/Calculation/Statistical/CHIDISTRightTail.php';
     }
 }
diff --git a/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiInvTest.php b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiInvRightTailTest.php
similarity index 59%
rename from tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiInvTest.php
rename to tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiInvRightTailTest.php
index 7268091493..afc4b023c7 100644
--- a/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiInvTest.php
+++ b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiInvRightTailTest.php
@@ -6,7 +6,7 @@
 use PhpOffice\PhpSpreadsheet\Calculation\Statistical;
 use PHPUnit\Framework\TestCase;
 
-class ChiInvTest extends TestCase
+class ChiInvRightTailTest extends TestCase
 {
     protected function setUp(): void
     {
@@ -18,14 +18,18 @@ protected function setUp(): void
      *
      * @param mixed $expectedResult
      */
-    public function testCHIINV($expectedResult, ...$args): void
+    public function testCHIINV($expectedResult, $probability, $degrees): void
     {
-        $result = Statistical::CHIINV(...$args);
+        $result = Statistical::CHIINV($probability, $degrees);
+        if (!is_string($expectedResult)) {
+            $reverse = Statistical\Distributions\ChiSquared::distributionRightTail($result, $degrees);
+            self::assertEqualsWithDelta($probability, $reverse, 1E-12);
+        }
         self::assertEqualsWithDelta($expectedResult, $result, 1E-12);
     }
 
     public function providerCHIINV()
     {
-        return require 'tests/data/Calculation/Statistical/CHIINV.php';
+        return require 'tests/data/Calculation/Statistical/CHIINVRightTail.php';
     }
 }
diff --git a/tests/data/Calculation/Statistical/CHIDIST2.php b/tests/data/Calculation/Statistical/CHIDISTLeftTail.php
similarity index 61%
rename from tests/data/Calculation/Statistical/CHIDIST2.php
rename to tests/data/Calculation/Statistical/CHIDISTLeftTail.php
index 5bc6761732..313061ac99 100644
--- a/tests/data/Calculation/Statistical/CHIDIST2.php
+++ b/tests/data/Calculation/Statistical/CHIDISTLeftTail.php
@@ -17,6 +17,30 @@
         0.776869839852,
         3, 2, true,
     ],
+    [
+        0.039646370521,
+        3, 9, false,
+    ],
+    [
+        0.035705027315,
+        3, 9, true,
+    ],
+    [
+        0.103349469094,
+        7.5, 8, false,
+    ],
+    [
+        0.516232618446,
+        7.5, 8, true,
+    ],
+    [
+        0.020666985354,
+        8, 3, false,
+    ],
+    [
+        0.953988294311,
+        8, 3, true,
+    ],
     [
         '#VALUE!',
         'NaN', 3, true,
diff --git a/tests/data/Calculation/Statistical/CHIDIST.php b/tests/data/Calculation/Statistical/CHIDISTRightTail.php
similarity index 100%
rename from tests/data/Calculation/Statistical/CHIDIST.php
rename to tests/data/Calculation/Statistical/CHIDISTRightTail.php
diff --git a/tests/data/Calculation/Statistical/CHIINV.php b/tests/data/Calculation/Statistical/CHIINVRightTail.php
similarity index 82%
rename from tests/data/Calculation/Statistical/CHIINV.php
rename to tests/data/Calculation/Statistical/CHIINVRightTail.php
index f931a78098..58b317e59e 100644
--- a/tests/data/Calculation/Statistical/CHIINV.php
+++ b/tests/data/Calculation/Statistical/CHIINVRightTail.php
@@ -10,13 +10,21 @@
         0.75, 10,
     ],
     [
-        18.30697345702,
-        0.050001, 10,
+        0.007716715545,
+        0.93, 1,
+    ],
+    [
+        1.021651247532,
+        0.6, 2,
     ],
     [
         0.45493642312,
         0.5, 1,
     ],
+    [
+        4.351460191096,
+        0.5, 5,
+    ],
     [
         0.101531044268,
         0.75, 1,

From 01a7b1965d5c969d43f81ff31f2a0cba8528b3e1 Mon Sep 17 00:00:00 2001
From: MarkBaker <mark@lange.demon.co.uk>
Date: Sun, 28 Mar 2021 15:41:17 +0200
Subject: [PATCH 6/8] Remove unused methods from Newton-Raphson

---
 .../Distributions/NewtonRaphson.php           | 33 ++-----------------
 1 file changed, 3 insertions(+), 30 deletions(-)

diff --git a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php
index 9a0ba4ed27..d4025f6f5b 100644
--- a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php
+++ b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php
@@ -10,44 +10,17 @@ class NewtonRaphson
 
     protected $callback;
 
-    protected $low = 100.0;
-
-    protected $high = 0.0;
-
-    protected $dx = 1;
-
     public function __construct(callable $callback)
     {
         $this->callback = $callback;
     }
 
-    public function setLow(float $lowValue): self
-    {
-        $this->low = $lowValue;
-
-        return $this;
-    }
-
-    public function setHigh(float $highValue): self
-    {
-        $this->low = $highValue;
-
-        return $this;
-    }
-
-    public function setDx(float $dxValue): self
-    {
-        $this->dx = $dxValue;
-
-        return $this;
-    }
-
     public function execute($probability)
     {
-        $xLo = $this->low;
-        $xHi = $this->high;
+        $xLo = 100;
+        $xHi = 0;
 
-        $dx = $this->dx;
+        $dx = 1;
         $x = $xNew = 1;
         $i = 0;
 

From 96e451fdedfe60569b4221dcfa7bd183d23dfbf0 Mon Sep 17 00:00:00 2001
From: MarkBaker <mark@lange.demon.co.uk>
Date: Sun, 28 Mar 2021 15:50:17 +0200
Subject: [PATCH 7/8] PHPCS

---
 .../Calculation/Statistical/Distributions/NewtonRaphson.php     | 2 +-
 .../Calculation/Functions/Statistical/ChiInvRightTailTest.php   | 2 ++
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php
index d4025f6f5b..6b2eb108d2 100644
--- a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php
+++ b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php
@@ -15,7 +15,7 @@ public function __construct(callable $callback)
         $this->callback = $callback;
     }
 
-    public function execute($probability)
+    public function execute($probability): float
     {
         $xLo = 100;
         $xHi = 0;
diff --git a/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiInvRightTailTest.php b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiInvRightTailTest.php
index afc4b023c7..75949f398b 100644
--- a/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiInvRightTailTest.php
+++ b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiInvRightTailTest.php
@@ -17,6 +17,8 @@ protected function setUp(): void
      * @dataProvider providerCHIINV
      *
      * @param mixed $expectedResult
+     * @param mixed $probability
+     * @param mixed $degrees
      */
     public function testCHIINV($expectedResult, $probability, $degrees): void
     {

From 0ec851e24b74b8068d434eddf7e85b6c3fef0d34 Mon Sep 17 00:00:00 2001
From: MarkBaker <mark@lange.demon.co.uk>
Date: Sun, 28 Mar 2021 16:01:54 +0200
Subject: [PATCH 8/8] Newton-Raphson can return a string if max iterations is
 reached

---
 .../Calculation/Statistical/Distributions/NewtonRaphson.php     | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php
index 6b2eb108d2..d4025f6f5b 100644
--- a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php
+++ b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php
@@ -15,7 +15,7 @@ public function __construct(callable $callback)
         $this->callback = $callback;
     }
 
-    public function execute($probability): float
+    public function execute($probability)
     {
         $xLo = 100;
         $xHi = 0;