While some rankings attempt to measure teaching using metrics such as staff to student ratio, the Higher Education Policy Institute has pointed out that the metrics used are more closely related to research than teaching quality, e.g. "Staff to student ratios are an almost direct measure of research activity", and "The proportion of PhD students is also to a large extent an indication of research activity". Inside Higher Ed similarly states "these criteria do not actually measure teaching, and none even come close to assessing quality of impact". Many rankings are also considered to contain biases towards the natural sciences and, due to the bibilometric sources used, towards publication in English-language journals. Some rankings, including ARWU, also fail to make any correction for the sizes of institutions, so a large institution is ranked considerably higher than a small institution with the same quality of research. Other compilers, such as Scimago and U.S. News and World Report, use a mix of size-dependent and size-independent metrics.