scdhlm provides a set of tools for estimating hierarchical linear
models and effect sizes based on data from single-case designs. The
estimated effect sizes, as described in Pustejovsky, Hedges, and Shadish
(2014), are comparable in principle to standardized mean differences
(SMDs) estimated from between-subjects randomized experiments. The
package includes functions for estimating design-comparable SMDs based
on data from treatment reversal designs with replication across
participants (Hedges, Pustejovsky, & Shadish, 2012), across-participant
multiple baseline designs and multiple probe designs (Hedges,
Pustejovsky, & Shadish, 2013; Pustejovsky, Hedges, & Shadish, 2014), and
more complex variations of multiple baseline designs (Chen, Pustejovsky,
Klingbeil, & Van Norman, 2023). Two estimation methods are available:
moment estimation and restricted maximum likelihood estimation. The
package also includes an interactive web interface implemented using
Shiny.
The development of this R package was supported in part by the Institute of Education Sciences, U.S. Department of Education, through Grant R324U190002 to the University of Oregon. The contents of the package do not necessarily represent the views of the Institute or the U.S. Department of Education.
Please cite this R package as follows:
Pustejovsky, J. E., Chen, M., & Hamilton, B. J. (2023). scdhlm: Estimating hierarchical linear models for single-case designs (Version 0.7.2) [R package]. https://CRAN.R-project.org/package=scdhlm
Please cite the web application as follows:
Pustejovsky, J. E., Chen, M., Hamilton, B., & Grekov, P. (2023). scdhlm: A web-based calculator for between-case standardized mean differences (Version 0.7.2) [Web application]. https://jepusto.shinyapps.io/scdhlm
You can install the released version of scdhlm from
CRAN with:
install.packages("scdhlm")You can install the development version from GitHub with:
# install.packages("remotes")
remotes::install_github("jepusto/scdhlm")You can access a local version of the interactive web-app by running the commands:
library(scdhlm)
shine_scd()Here we demonstrate how to use scdhlm to calculate design-comparable
SMDs based on data from different single-case designs. We will first
demonstrate the recommended approach, which uses restricted maximum
likelihood (REML) estimation. We will then demonstrate the older, moment
estimation methods. The moment estimation methods were the originally
proposed approach (described in Hedges, Pustejovsky, & Shadish, 2012,
2013). The package provides these methods for sake of completeness, but
we no longer recommend them for general use.
Laski, Charlop, and Schreibman (1988) used a multiple baseline across individuals to evaluate the effect of a training program for parents on the speech production of their autistic children, as measured using a partial interval recording procedure. The design included eight children. One child was measured separately with each parent; following Hedges, Pustejovsky, and Shadish (2013), only the measurements taken with the mother are included in our analysis.
For this study, we will estimate a design-comparable SMD using
restricted maximum likelihood (REML) methods, as described by
Pustejovsky and colleagues (2014). This is a two-step process. The first
step is to estimate a hierarchical linear model for the data, treated
the measurements as nested within cases. We fit the model using
nlme::lme()
library(nlme)
library(scdhlm)
data(Laski)
# Pustejovsky, Hedges, & Shadish (2014)
Laski_RML <- lme(fixed = outcome ~ treatment,
random = ~ 1 | case,
correlation = corAR1(0, ~ time | case),
data = Laski)
Laski_RML
#> Linear mixed-effects model fit by REML
#> Data: Laski
#> Log-restricted-likelihood: -519.1424
#> Fixed: outcome ~ treatment
#> (Intercept) treatmenttreatment
#> 39.07612 30.68366
#>
#> Random effects:
#> Formula: ~1 | case
#> (Intercept) Residual
#> StdDev: 15.68278 13.8842
#>
#> Correlation Structure: AR(1)
#> Formula: ~time | case
#> Parameter estimate(s):
#> Phi
#> 0.252769
#> Number of Observations: 128
#> Number of Groups: 8The summary of the fitted model displays estimates of the component parameters, including the within-case and between-case standard deviations, auto-correlation, and (unstandardized) treatment effect estimate. These estimated components will be used to calculate the effect size in next step.
The estimated variance components from the fitted model can be obtained
using extract_varcomp():
varcomp_Laski_RML <- extract_varcomp(Laski_RML)
varcomp_Laski_RML
#> $Tau
#> $Tau$case
#> case.var((Intercept))
#> 245.9497
#>
#>
#> $cor_params
#> [1] 0.252769
#>
#> $var_params
#> numeric(0)
#>
#> $sigma_sq
#> [1] 192.7711
#>
#> attr(,"class")
#> [1] "varcomp"The estimated between-case variance is 245.95, the estimated auto-correlation is 0.253, the estimated and the estimated within-case variance is 192.771. These estimated variance components will be used to calculate the effect size in next step.
The second step in the process is to estimate a design-comparable SMD
using scdhlm::g_mlm(). The SMD parameter can be defined as the ratio
of a linear combination of the fitted model’s fixed effect parameters
over the square root of a linear combination of the model’s variance
components. g_mlm() takes the fitted lme model object as input,
followed by the vectors p_const and r_const, which specify the
components of the fixed effects and variance estimates that are to be
used in constructing the design-comparable SMD. Note that r_const is a
vector of 0s and 1s which specify whether to use the variance component
parameters for calculating the effect size: random effects variances,
correlation structure parameters, variance structure parameters, and
level-1 error variance. The function calculates an effect size estimate
by first substituting maximum likelihood or restricted maximum
likelihood estimates in place of the corresponding parameters, then
applying a small-sample correction. The small-sample correction and the
standard error are based on approximating the distribution of the
estimator by a t distribution, with degrees of freedom given by a
Satterthwaite approximation (Pustejovsky, Hedges, & Shadish, 2014). The
g_mlm() function includes an option allowing use of the expected or
average form of the Fisher information matrix in the calculations.
In this example, we use the treatment effect in the numerator of the
effect size and the sum of the between-case and within-case variance
components in the denominator of the effect size. The constants are
therefore given by p_const = c(0, 1) and r_const = c(1, 0, 1). The
effect size estimated is calculated as:
Laski_ES_RML <- g_mlm(Laski_RML, p_const = c(0, 1), r_const = c(1, 0, 1))
Laski_ES_RML
#> est se
#> unadjusted effect size 1.465 0.299
#> adjusted effect size 1.405 0.286
#> degree of freedom 18.552The adjusted SMD effect size estimate is 1.405 with standard error of 0.286 and degree of freedom 18.6.
A summary() method is included, which returns more detail about the
model parameter estimates and effect size estimate when setting
returnModel = TRUE (the default) in g_mlm():
summary(Laski_ES_RML)
#> est se
#> Tau.case.case.var((Intercept)) 245.950 142.179
#> cor_params 0.253 0.100
#> sigma_sq 192.771 28.265
#> total variance 438.721 144.047
#> (Intercept) 39.076 5.989
#> treatmenttreatment 30.684 2.996
#> treatment effect at a specified time 30.684 2.996
#> unadjusted effect size 1.465 0.299
#> adjusted effect size 1.405 0.286
#> degree of freedom 18.552
#> constant kappa 0.143
#> logLik -519.142The CI_g() calculates a symmetric confidence interval using a central
t distribution (the default) or an asymmetric confidence interval using
non-central t distribution (setting symmetric = FALSE).
CI_g(Laski_ES_RML)
#> [1] 0.8046224 2.0051521
CI_g(Laski_ES_RML, symmetric = FALSE)
#> [1] 0.9143684 2.0046719The symmetric confidence interval is [0.805, 2.005] and the asymmetric confidence interval is [0.914, 2.005].
Lambert, Cartledge, Heward, and Lo (2006) tested the effect of using
response cards (compared to single-student responding) during math
lessons in two fourth-grade classrooms. The investigators collected data
on rates of disruptive behavior and academic response for nine focal
students, using an ABAB design. This example is discussed in Hedges,
Pustejovsky, and Shadish (2012), who selected it because the design was
close to balanced and used a relatively large number of cases. Their
calculations can be replicated using the effect_size_ABk() function.
To use this function, the user must provide the names of five variables:
- the outcome variable,
- a variable indicating the treatment condition,
- a variable listing the case on which the outcome was measured,
- a variable indicating the phase of treatment (i.e., each replication of a baseline and treatment condition), and
- a variable listing the session number.
In the Lambert dataset, these variables are called respectively
outcome, treatment, case, phase, and time. Given these inputs,
the design-comparable SMD is calculated as follows for the measure of
academic response:
data(Lambert)
Lambert_academic <- subset(Lambert, measure == "academic response")
Lambert_ES <- effect_size_ABk(outcome = outcome, treatment = treatment, id = case,
phase = phase, time = time, data = Lambert_academic)
Lambert_ES
#> est se
#> unadjusted effect size 7.171 0.484
#> adjusted effect size 7.128 0.482
#> degree of freedom 126.671The adjusted effect size estimate delta_hat is equal to 7.128; its
variance V_delta_hat is equal to 0.232. A standard error for
delta_hat can be calculated by taking the square root of
V_delta_hat: sqrt(Lambert_ES$V_delta_hat) = 0.482. The effect size
estimate is bias-corrected in a manner analogous to Hedges’ g correction
for SMDs from a between-subjects design. The degrees of freedom nu are
estimated based on a Satterthwaite-type approximation, which is equal to
126.7 in this example.
A summary() method is included to return more detail about the model parameter estimates and effect size estimates:
summary(Lambert_ES)
#> est se
#> within-case variance 0.018
#> sample variance 0.014
#> intra-class correlation 0.000
#> auto-correlation 0.091
#> numerator of effect size estimate 0.858
#> unadjusted effect size 7.171 0.484
#> adjusted effect size 7.128 0.482
#> degree of freedom 126.671
#> scalar constant 0.166Saddler, Behforooz, and Asaro (2008) used a multiple baseline design to investigate the effect of an instructional technique on the writing of fourth grade students. The investigators assessed the intervention’s effect on measures of writing quality, sentence complexity, and use of target constructions.
Design-comparable SMDs can be estimated based on these data using the
effect_size_MB() function. The following code calculates a
design-comparable SMD estimate for the measure of writing quality:
data(Saddler)
Saddler_quality <- subset(Saddler, measure=="writing quality")
quality_ES <- effect_size_MB(outcome, treatment, case, time, data = Saddler_quality)
quality_ES
#> est se
#> unadjusted effect size 2.149 0.634
#> adjusted effect size 1.963 0.579
#> degree of freedom 8.918The adjusted effect size estimate delta_hat is equal to 1.963, with
sampling variance of V_delta_hat equal to 0.335 and a standard error
of 0.579.
summary(quality_ES) returns more detail about the model parameter
estimates and effect size estimates:
summary(quality_ES)
#> est se
#> within-case variance 0.349
#> sample variance 0.952
#> intra-class correlation 0.633
#> auto-correlation 0.100
#> numerator of effect size estimate 2.097
#> unadjusted effect size 2.149 0.634
#> adjusted effect size 1.963 0.579
#> degree of freedom 8.918
#> scalar constant 0.201Chen, M., Pustejovsky, J. E., Klingbeil, D. A., & Van Norman, E. R. (2023). Between-case standardized mean differences: Flexible methods for single-case designs. Journal of School Psychology, 98, 16-38. https://doi.org/10.1016/j.jsp.2023.02.002
Gilmour, A. R., Thompson, R., & Cullis, B. R. (1995). Average information REML: An efficient algorithm for variance parameter estimation in linear mixed models. Biometrics, 51(4), 1440–1450. https://doi.org/10.2307/2533274
Hedges, L. V., Pustejovsky, J. E., & Shadish, W. R. (2012). A standardized mean difference effect size for single case designs. Research Synthesis Methods, 3(3), 224-239. https://doi.org/10.1002/jrsm.1052
Hedges, L. V., Pustejovsky, J. E., & Shadish, W. R. (2013). A standardized mean difference effect size for multiple baseline designs across individuals. Research Synthesis Methods, 4(4), 324-341. https://doi.org/10.1002/jrsm.1086
Lambert, M. C., Cartledge, G., Heward, W. L., & Lo, Y. (2006). Effects of response cards on disruptive behavior and academic responding during math lessons by fourth-grade urban students. Journal of Positive Behavior Interventions, 8(2), 88-99. https://doi.org/10.1177/10983007060080020701
Laski, K. E., Charlop, M. H., & Schreibman, L. (1988). Training parents to use the natural language paradigm to increase their autistic children’s speech. Journal of Applied Behavior Analysis, 21(4), 391–400. https://doi.org/10.1901/jaba.1988.21-391
Pustejovsky, J. E., Hedges, L. V., & Shadish, W. R. (2014). Design-comparable effect sizes in multiple baseline designs: A general modeling framework. Journal of Educational and Behavioral Statistics, 39(4), 211-227. https://doi.org/10.3102/1076998614547577
Saddler, B., Behforooz, B., & Asaro, K. (2008). The effects of sentence-combining instruction on the writing of fourth-grade students with writing difficulties. The Journal of Special Education, 42(2), 79–90. https://doi.org/10.1177/0022466907310371