Using EXCEL to obtain a 95% bootstrap confidence interval for a mediation effect, ab

This [attachment:boot.doc word document] gives details of how to obtain a bootstrap confidence interval for ab. Bootstrapping has the advantage of not assuming normality ie not requiring a large sample. An example file is given [attachment:booteg.xls here.] This includes input data in sheets 1 and 2 and a bootstrap output spreadsheet (Bootraw4) containing results of estimating ab from 1000 bootstrap samples. In this example the bootstrap 95% confidence interval given in the Bootraw4 spreadsheet is (-1.56, 0.4) suggesting there is no statistically significant mediation effect. A bootstrapped 95% confidence interval for the standardized effect size of Preacher and Kelley (2011) may be computed using [attachment:mab.xls this spreadsheet]. In the example in this spreadsheet the bootraw6 sheet contains the 95% confidence interval of (0.009, 0.39) suggesting a large mediation effect.

To use these spreadsheets you will first have to download a bootstrap macro from [http://www3.wabash.edu/econometrics/EconometricsBook/Basic%20Tools/ExcelAddIns/bootstrap.htm this website] onto your PC to be added into EXCEL (instructions are given on the website and are also given in the Word document [attachment:bootmac.doc here).]

Note: You may find that that you cannot update cells in the spreadsheet with new data after you have run the bootstrap macro. This may be rectified by clicking on the office button (top left corner of spreadsheet) which has red, gold, blue and green squares on it, clicking the 'EXCEL OPTIONS' button in the bottom middle, 'FORMULAS' (second from top in left hand menu) and selecting 'AUTOMATIC EXCEPT FOR DATA TABLES' in the 'Calculation options' menu at the top of the resultant window and, finally, 'OK'.

Reference

Preacher KJ and Kelley K (2011). Effect size measures for mediation models: quantitative strategies for communciating indirect effects. Psychological Methods 16(2) 93-115.