The ultimate tip that I have would be to pack a snack and a bottle of water just for the test.
Background.
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I intentionally didn't research what an IQ test involved because DW and I did not want to prime our DD and we wanted the results to be as 'honest' as possible. We were not testing for a GT program and were not expecting our DD to be found to be well into the 'GT' zone but were testing to rule this out as an outside possibility for the cause of behavioural issues that we were observing in our daughter. As a result, I naively thought that we would be done after 30 mins or so. Instead, it took about 2-3 hours and I saw my DD flag a bit towards the end, had she had a snack and a drink I honestly think she would have done better on the WMI and PSI tests.