Complex sentence: They noted that the data taken of adoption placement effects can explain the observed differences; but that they cannot make that claim firmly because the pre-adoption factors confounded racial ancestry, preventing an unambiguous interpretation of the results.
Split into simpler sentences: They noted that the data taken of adoption placement effects can explain the observed differences. But that they cannot make that claim firmly due to the pre adoption factors confounded racial ancestry, preventing an unambiguous interpretation of the results.