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.
A common way of assessing the reliability of observations is to use inter-rater reliability. This involves comparing the ratings of two or more observers and checking for agreement in their measurements.
Another way of improving the reliability of an observational study is to ensure that the categories are clearer.
This refers to whether a study measures or examines what it claims to measure or examine. Observations are said to often lack validity for a number of reasons. If participants are aware they are being observed they may behave in the way they feel they should behave. Perhaps some of the categories could have been coded in a different way.