Reliability: Its the extent to which the way in which you measure a vairable or concept is consistent so repeating measures would produce similar results.
In the context of an Experiment, reliability refers to the ability to repeat a study and replicate it and produce similar results with conditions all being the same.
In an Observation, two observers should produce the same results and the extent to which the observers agree is called Inter-Observer reliability. To improve inter-observer reliablity, observers should be trained carefully in the coding system so that they are both looking for the exact same thing.
Internal Reliability: Its the level at which something is consistent within itself for example questions in an IQ test should all be measuring the same thing.
External Reliability: Its the measure of consistency over several occasions so say an experiment was repeated several times over a few months, if they all produced similar results then it has high external reliability.
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