Stratified sampling is the most suitable technique for selecting customers for testing the IT support department's success in meeting service levels, as it involves dividing the population into distinct subgroups (strata) based on certain characteristics (in this case, customer size classification based on annual expenditures and service nature). This method ensures that each subgroup is adequately represented in the sample, providing more reliable and relevant results. Reference:
The IIA's Global Technology Audit Guide (GTAG) on Data Analysis Technologies.
The IIA's Practice Guide on Audit Sampling.