The internal consistency reliability of survey instruments (e.g. psychological tests), is a measure of reliability of different survey items intended to measure the same characteristic.
For example, there are 5 different questions (items) related to anxiety level. Each question implies a response with 5 possible values on a Likert scale, e.g. scores -2,-1,0,1,2. Responses from a group of respondents have been obtained. In reality, answers to different questions vary for each particular respondent, although the items are intended to measure the same aspect or quantity. The smaller this variability (or stronger the correlation), the greater the internal consistency reliability of this survey instrument.
The split-halves method is often used to measure the internal consistency reliability of survey instruments.
Survey Analysis,
Survey Design and Sampling Procedures