1. Reliability Internal External Test-retest Inter-rater
Reliability refers to how consistent or dependable a test is.
A reliable test carried out in the same circumstances on the same participants should always give the same results.
There are different types of reliability;
Internal
External
Test-retest
Inter-rater

2. Internal Reliability
Different parts of the test should give consistent results.
For example, if an lQ test contains sections of supposedly equal difficulty, participants should achieve similar scores on all sections.
External Reliability The test should produce consistent results regardless of when it’s used.
For example, if you took the same IQ test on two different days you should achieve the same store.
Test-retest Reliability
The test should produce the same results when the same participants repeat it.
Test-retest is a way of testing external reliability.
Inter-rater Reliability The test should give consistent results regardless of who administers it.
For example, if two researchers observe behaviour and categorise aggression scores in children, they should both record the same score.

3. Increasing Reliability
Reliability in a study can be increased by doing the following:
Standardising the procedure to ensure it is controlled
Clearly operationalising variables
Conducting pilot studies
Controlling as many variables as possible
Using an artificial environment

4. Validity
Validity refers to how well a test measures what it claims to. For example, an IQ test with only maths questions would not be a valid measure of general intelligence. There are different types:

5. Face validity the extent to which the test looks, to the participants, like it will measure what it is supposed to be measuring.
Concurrent validity the extent to which the test produces the same results as another established measure, e.g. two different IQ tests should produce the same measure of IQ.
Ecological validity the extent to which the setting/environment of the test reflect real-life.
Construct validity the extent to which the tasks involved in the study test reflect real-life.
Internal validity the extent to which the outcome of the study is the direct result of the manipulated independent variable.
External validity the extent to which the findings apply to other people and situations
Predictive validity the extent to which results from a study or a test can predict future behaviour
Population validity the extent to which the findings can be applied to other people

6. Remember: Being reliable is being consistent
Remember: Being reliable is being consistent. Being valid is being on target.

7. Reliable, but not valid
Not reliable, not valid
Reliable and valid