Reliability and validity

When we develop questionnaires, it is important to ensure that the data collected is reliable and valid. Reliability refers to the extent to which a measure is free from random error and therefore provides consistent results. Validity indicates how well a question measures what it is specifically designed to assess.

There are different sources of errors and corresponding estimates of reliability as presented in Table 9.2:

Source of error	What are we asking?	Estimate of reliability	How to calculate
Time-specific	Is there stability in customer satisfaction over time	Test-retest	Correlation between the same survey on different occasions
Item-specific	Can the items generalize to...