Reliability and Validity
The centre of the target is the concept that you are trying to measure - If you measure the concept perfectly for a person, you are hitting the centre of the target
1 you are consistently and systematically measuring the wrong value for all respondents
2 you get a valid group estimate, but you are inconsistent (on average, you are getting the right answer for the group (but not very well for individuals)
3 Neither reliable nor valid
4 Both reliable and valid
Adapted from Trochim, 2002