Reliability and Validity
Reliability refers to how consistent a measuring device is. A measurement is said to be reliable or consistent if the measurement can produce similar results if used again in similar circumstances. For example, if a speedometer gave the same readings at the same speed it would be reliable. If it didn't it would be pretty useless and unreliable.
Importantly reliability of self-report measures, such as psychometric tests and questionnaires can be assessed using the split half method. This involves splitting a test into two and having the same participant doing both halves of the test. If the two halves of the test provide similar results this would suggest that the test has internal reliability.
This refers to whether a study measures or examines what it claims to measure or examine. Questionnaires are said to often lack validity for a number of reasons. Participants may lie; give answers that are desired and so on. It is argued that qualitative data is more valid than quantitative data.