I'm writing up an ERA, where the participants were shown a bunch of unambiguous images, which morphed into an ambiguous figure to test if past experience affects visual perception. There was two experimental groups and no control groups, where one group started off with the unambiguous image of a man and the other a woman and they morphed into the same ambiguous image. Participant who said that the ambiguous image was the same as the first unambiguous image they saw indicated that past experience did affect visual perception.
I said that the independent variable was 'the set of 3 visual stimuli images shown before the ambiguous figure'. However, I got this incorrect as it 'should have been an operational independent variable and what you said wasn't measurable, hence not operational'. I can't figure out how we can measure it, it's not like the dependent variable where you can count the number of people who had results consistent with the unambiguous image. Anyone have any idea what it could be?