In psychometric surveys, the split-halves method is used to measure the internal consistency reliability of survey instruments, e.g. psychological tests.
The idea is to split the items (questions) related to the same construct to be measured, e.d. the anxiety level, and to compare the results obtained from the two resulting subsets of items. The closer the results - i.e. the scores of the construct being measured (e.g the anxiety level), the greater the internal consistency reliability of this survey instrument.
The correlation coefficient between the two sets of measurements is often used as a quantitative measure of the internal consistency reliability of a survey instrument.
Related statistics.com courses:
Survey Design and Sampling Procedures