To be sure, technology advances and differences between travel and retail curators make direct comparisons difficult.
(Because the relevant studies had different follow-up periods, it's hard to make direct comparisons).
Because different exchanges measure their trading volume in different ways, it is difficult to make direct comparisons.
The figures put together by the magazine are intended to help consumers make direct comparisons between retailers.
Voters have barely begun to look at Mr Kerry, let alone make direct comparisons with Mr Bush.
It is difficult to make direct comparisons; defining what makes an "athlete," for example, is problematic, and police and judicial procedures vary from county to county.
Because the form factor is so different, people don't seem to make direct comparisons with the desktop PC, and therefore don't expect the user experience to be identical.
Coupled with this, the small number of housing transactions taking place means it is not always easy to make direct comparisons with similar properties recently sold.
Kieron Broadhead, head of marketing and recruitment at Goldsmiths, University of London, agrees: "By attending open days, students are able to make direct comparisons between each institution.
Laithwaites cleverly used its own labels so that punters couldn't make direct comparisons with the same wines on other people's websites.
Netflix closely guards its viewership data, so it is hard to make direct comparisons to broadcast and cable networks, which allow themselves to be rated.
