Most surveys conducted by the secular community confuse “Jews” with persons of Jewish descent. One should definitely exclude those persons of Jewish descent who don’t have a clear claim of being Jewish based on matrilineal descent. One should probably exclude those who have been so assimilated that they are not distinguishable from the general population (I suggest such matters as observance of Shabbos and Kashrus are key factors in deciding whether one has assimilated).
If you found some DNA from Sinai (when the Jewish people began), you would probably discover that almost everyone is of Jewish descent (which would be true of anyone from 3500 years ago – at this point all humans are related to all others-remember Noach is about 4500 years ago, and we hold everyone is descended from him).