The phrase "a reference is needed" is correct and usable in written English.
It can be used when indicating that a citation or source of information is required for clarity or validation.
Example: "In order to support your argument, a reference is needed to substantiate your claims."
Alternatives: "a citation is required" or "a source is necessary."
If so a reference is needed.
It is only in the last step while calculating relative CNs that a reference is needed.
A reference is needed for the statement about M. buccinatorius having many patterns among mammals.
Regarding the section on mandibular dentition: a reference is needed for the statement that large interdental spaces are unusual in herbivores, and for the statement that unfused zygomatic arches distinguish pilosans from most other mammals.
However, to create a SNP tree, a good reference is needed and it is also frequently necessary to sort out false SNPs.
A healthy reference is needed in order to determine how these relationships compare or change under osteoarthritic conditions.
Therefore, we test innate preference (see next paragraph, Innate preference tests) between a container with 10 μL of PAR versus an empty container (EM); larvae behave indifferently in this situation (supplementary Figure 1A; one-sample sign [OSS] test: P = 0.791, N = 16); we therefore use the EM condition in all cases where a no-odor reference is needed.
To facilitate the use of biocoenotical data at the landscape level, a standard frame of reference is needed.
A broader frame of reference is needed: one that links together WHO's work in tobacco, nutrition and physical activity, and even alcohol, and that envisages the strategic use of international legal standards, non-binding international recommendations, advocacy and policy advice as tools of choice for promoting different elements of the strategy.
"E2F7/8 family was not as indispensable as E2F1-6, RB, and DP family" if it is not your conclusion, reference is needed.
In order to make comparisons of protein abundance, a common reference was needed.
