Pretest-to-posttest change in a Solomon design — plot_solomon

Draws mean pretest and posttest scores for the two pretested groups, joined to show change, alongside posttest means for the two unpretested groups. The unpretested groups appear at posttest only: their missing pretest is the experimental manipulation (Solomon, 1949), not missing data, and the figure labels it that way.

Usage

plot_solomon_change(
  y_post,
  treat,
  pretested,
  y_pre,
  show_individuals = FALSE,
  conf_level = 0.95
)

Arguments

y_post: Numeric posttest scores.
treat: Treatment indicator coded 0 = control and 1 = treatment.
pretested: Pretest indicator coded 0 = unpretested and 1 = pretested.
y_pre: Numeric pretest scores, missing by design for unpretested participants.
show_individuals: Logical; if TRUE, draw each pretested participant's change as a faint line behind the means. Default FALSE.
conf_level: Confidence level for the intervals, which use the t distribution with n - 1 degrees of freedom. Default is 0.95.

Value

A ggplot object.

Details

Trajectories use pretested participants with both scores observed. Pretested participants whose pretest is incidentally missing are left out of the trajectories and counted in the caption, and are never imputed; see check_solomon_missing().

A change reading of this figure applies only to the pretested groups. The Solomon contrasts the package reports compare posttests, adjusting for the pretest where one exists; see compare_solomon_methods() for the estimand behind each analysis.

References

Solomon, R. L. (1949). An extension of control group design. Psychological Bulletin, 46(2), 137-150.

Examples

with(solomon_example, plot_solomon_change(y_post, treat, pretested, y_pre))