Sample sizes (MR, unit 9)

No universal law for determining, rely on experience
Larger the sample size, the more accurate the results
Point at which increasing sample size has little impact on results

Consider:

• money & time available
• degree of precision required (e.g. road widening scheme v. drug testing)
• number of subsamples required - stratified sampling needs large overall size to ensure adequate representation of each strata

Minimum sample sizes: 300-500
Less than 50 ONLY for qualitative data
Any subgroup of less than 100 may not be statistically valid
National surveys for consumer goods use sample size: 1500-200