Research that compares those who do and do not participate in protest over time purports that protesters are becoming increasingly similar to the non-protesting population. Using a protest survey that includes the frequency of protest participation, we consider the extent to which those who protest to different degrees are similar to non-protesters. Selection bias in non-probability protest survey data is compensated for by combining the data with random reference samples from the European Social Survey under a quasi-randomisation approach. We test hypotheses on the normalization of protesters and compare two methods for compensating for selection bias: a proportional weighting method and a propensity score adjustment method. The propensity score adjustment method is more effective in mitigating selection bias by balancing on variables that explain the selection and outcome, and enables the comparison of groups of protesters to non-protesters. We find that protesters become increasingly differentiated from non-protesters according to their left-wing self-placement and education as their extent of protest participation increases.