Descriptive assumption diagnostics for Solomon analyses

Reports Brown-Forsythe tests of equal posttest variance (Brown & Forsythe, 1974), Shapiro-Wilk normality tests within cells, and a test of homogeneous pretest-posttest slopes among pretested participants with complete scores.

Usage

check_solomon_assumptions(y_post, treat, pretested, y_pre)

Arguments

y_post

numeric posttest

treat

0/1 (or logical) treatment indicator

pretested

0/1 (or logical) pretest indicator

y_pre

numeric pretest (NA for unpretested)

Value

An object of class solomon_checks.

Details

These results are descriptive. Choosing an analysis according to whether a preliminary assumption test is significant can distort Type I error rates (Zimmerman, 2004), so solomonR does not use them as gates: heteroskedasticity-consistent standard errors are a reasonable default for the unified GLM regardless of these results (Long & Ervin, 2000). A small slope-homogeneity p-value is substantively informative: it suggests that the treatment effect among pretested participants depends on the pretest score.

References

Brown, M. B., & Forsythe, A. B. (1974). Robust tests for the equality of variances. Journal of the American Statistical Association, 69(346), 364-367.

Long, J. S., & Ervin, L. H. (2000). Using heteroscedasticity consistent standard errors in the linear regression model. The American Statistician, 54(3), 217-224.

Zimmerman, D. W. (2004). A note on preliminary tests of equality of variances. British Journal of Mathematical and Statistical Psychology, 57(1), 173-181.