'be difficult to correlate' is a correct and usable sentence in written English.
You could use it to describe something that is challenging to find a connection between. For example: "The data collected is so varied that it can be difficult to correlate."
Furthermore, it would be difficult to correlate in vitro testing with in vivo conditions, which most likely involve multiple environmental factors that affect enterotoxin gene regulation.
At this resolution it would be difficult to correlate the PET signal with individual stenoses or to determine which regions, specifically, are at risk to rupture.
It also demonstrates that it can be difficult to correlate demographic data inferred from phylogenies with large-scale environmental fluctuations, due to the considerable statistical uncertainty that is often associated with intraspecific estimates of divergence times.
Fluctuations such as this would be difficult to correlate with any other measure.
Without long-term follow-up and additional larger studies it will be difficult to correlate the findings in this study with the onset and/or prevalence of other chronic diseases.
At present, it would be difficult to correlate the increased VEGFR1 surface levels to increased second messenger signaling, as VEGFR1 signals through the PI3K/Akt and phospholipase C gamma/protein kinase C pathways 98, which are constitutively active in breast cancer cell lines, xenografts, and tissue 66, 99– 102.
Because of this, it has often been difficult to correlate rock strata with layers from other localities.
The different literature reports are difficult to correlate due to the large variances.
As tantalizing as the rankings may be, it is difficult to correlate them to the number of books sold, unless you are J.
Some of these can be well correlated to the standard European ammonite biozones; however, many stratigraphic sequences in Asia and North America are difficult to correlate globally.
In addition, because of the limited geographic range of many species, it is difficult to correlate strata between regions of the world.
