We generally recommend using the results from the person's first questionnaire they've taken. Most of the time, we find it is the most accurate.
If a person received interpretation on the first report prior to taking the second questionnaire, the interpretation can affect the person's second responses.
Many times the second report will not have significantly different results. If a person has high scores on Social Energy (formerly Acceptance) Usual Behavior, it is likely to remain high on the second report. The actual numbers may change, but both will be high. This is another reason we will most often use the first report.
It is important to remember that The Birkman Method is a wonderful instrument, but it does not measure everything. There will be issues surrounding morals, values, etc. that will not be measured. The information not reported can affect how certain scores will show themselves in a person's actions.
Also, there are times when an individual disagrees with one or two scores on the report. In this case you may want to ask the person to let someone close (spouse, etc.) read the results for that score. If the second person also disagrees with the score, then we, as interpreters, need to recommend that the person disregard that particular score. It does not mean that the entire report is invalid. No assessment tool will be 100% accurate all of the time.