Making _respectively_, labels and comparisons explicit
Respectively pairs the items of one list with those of another, and a comparison sets one thing against another. Both are clear only when the reader sees at once what goes with what: respectively helps in some sentences and not in others, a value can be written beside its label instead, and a comparison needs both of its sides in view.
What respectively asks of the reader
Few words cause as much confusion in science as the adverb respectively. It is frequently misused by native and non-native English-speaking authors alike, and its misuse can lead to confusion and ambiguities. The main reason is that other languages have a word that resembles respectively but may mean something else: the German respektive usually simply means or, and respectively is then inappropriate.
Respectively is widely used in research writing, probably for concision, but it induces backtracking: the reader has to match each member of one set of words, usually nouns, to each member of a prior set. It asks the reader to relate two or more separated elements, usually variables and numbers, that appear before it. Looking back to check which variable goes with which number almost always disturbs the reader's concentration on the argument of the text, even if the statement is short. One statement of results, comparing reductions in cholesterol with percentages, confidence intervals and P values in nested parentheses and ending with respectively, is exceedingly uncomfortable to read, partly because of the array of parentheses but mainly because of the respectively. A writer who puts respectively at the end of a statement should ask whether the reader will be able to remember instantly which previous words it refers back to.
When respectively helps
Respectively should be used only in sentences where there is a clear relationship between pairs of variables. A sentence without it is often clearer, but where a sentence contains two corresponding lists, respectively may be useful to economise on words, as in The mean time to disease progression in groups 1 and 2 was 5 weeks and 9 weeks, respectively.
For a term shorter than the time to disease progression, such as average weights, respectively is not necessary. Mean body weight was 72 kg in group 1 and 83 kg in group 2 is preferable to a construction with respectively, because it is shorter and readily understood anyhow. So respectively should be used sparingly, only where it helps to clarify the relationships, and not at all in a sentence that is clear without it.
Respectively is also misused where there is no second set at all. A sentence reporting that the plaque index and gingival bleeding index were significantly reduced, respectively, over six weeks has one set of indexes and no second set to match them against, so respectively is not needed; the set of indexes probably prompted the mistake, and the suggested revision uses the determiner each: the two indexes were each significantly reduced.
Writing each value beside its label
In The mean specific radioactivity in lungs and plasma of the rats was 16 and 18 DPM/ng, respectively, the reader has to match a pair of values to a pair of tissues, which takes extra cognitive effort, and alerting the reader with respectively does not excuse the writer from facilitating comprehension. The suggested revision sets each value beside its tissue, in the order in which the original sentence named the tissues: The mean specific radioactivity of the rat tissues was 16 DPM/ng (lung) and 18 DPM/ng (plasma). Sensitivity, specificity, PPV, and NPV of NMP22 to detect recurrent bladder carcinoma in the total group of patients was, respectively, 50, 68, 22, and 89% takes 25 words, while In the total group of patients, NMP22 detected recurrent bladder carcinoma with 50% sensitivity, 68% specificity, 22% PPV, and 89% NPV takes 21 and is easier to process.
Respectively is among the reader-unfriendly terms one writer advises excising, and a clumsy sentence that uses it can be rewritten to name each pairing outright. He connected the renal vessels of a sheep and a pig kidney, respectively, to the brachial vessels of two patients becomes he connected the renal vessels of a sheep kidney to the brachial vessels of one of the patients and a pig kidney to the brachial vessels in the other patient. Lymphocytes require synthesis of purine and pyrimidine nucleotides for replication, regulated by inosine monophosphate dehydrogenase (IMPDH) and dihydroorotate dehydrogenase (DHODH), respectively becomes To replicate, lymphocytes require synthesis of purine nucleotides, regulated by inosine monophosphate dehydrogenase (IMPDH), and of pyrimidine nucleotides, regulated by dihydroorotate dehydrogenase (DHODH).
A careless respectively makes comprehension more laborious and time-consuming, if not impossible, for the typically overwhelmed reader. Two sentences from otherwise well-written articles in the normally well-edited New England Journal of Medicine show it: increases in local and regional disease in the late 1970s and early 1980s and again in the late 1990s "may reflect the increasing use of sigmoidoscopy and colonoscopy, respectively", and two population groups "account for 56% and 26% of Hawaii's population, respectively". In both, the author or editor should have done the matching, so that the reader need not backtrack to work out what goes with what.
Big lists of mix-and-match items joined by respectively, such as The hazard ratios for groups A, B, C, D, and E were 1.82, 2.2, 1.54, 4.5, and 0.95, respectively, or adverse-event rates for community and nursing-home residents across three therapies, strike one editor of research-article summaries as confusing and easy to misread, and that editor would like to give writers rules, perhaps avoiding comparing more than three items or more than two lists. Respectively should not be used in such a context: it is much more convenient for the reader to have things spelled out and intelligible at first reading. The first can be spelled out as a series that repeats each group beside its value, and the second as explicit vs. comparisons that name both populations for each therapy.
Completing a comparison
Than in and than those in
In The time-weighted average leukocyte counts were higher in group X than that in group Y, that is grammatically incorrect, because it refers back to a plural noun, counts, in the subject; than those in group Y is fine. It is unlikely that any reader would misunderstand the sentence whether it ends than group Y, than in group Y or than those in group Y. Leaving out in before group Y seems acceptable here, because group X immediately precedes group Y in the same adverbial phrase and the prepositional effect of in is easily transferred to group Y when reading, as it often is in short adverbial phrases, particularly in speech.
Putting the verb between the two groups makes it unacceptable to leave out in: with The time-weighted average leukocyte counts in group X were higher than group Y, the reader has the feeling that something is missing. The prepositional effect of in is not transferred, because the verb splits two separate adverbials and the reader has to contend with the verb before reading about group Y, so with this word order the choice should be than in group Y or than those in group Y. With the groups further apart still, as in The group X time-weighted average leukocyte counts were higher than group Y, leaving out in can be discounted for the same reasons. In that word order, than in group Y is just about all right but leaves the groups too far apart to be comfortable, and than those in group Y definitely contributes to reader comfort and comprehension. Most writers would probably choose either of these two only because of word-count constraints, as they are certainly the least elegant.
Some prescriptivists would insist that higher in group X than those in group Y and in group X were higher than those in group Y are the most correct, because those makes it absolutely clear that the counts in both groups are meant. One experienced editor, who always applies the test "Can this sentence be misunderstood?", finds that higher in group X than in group Y and in group X were higher than in group Y cannot be misunderstood and are good enough. Every situation still has to be assessed individually by the author or editor, and there may well be instances where the additional security of those is useful for comprehension or reader comfort. That editor's order of preference runs: higher in group X than in group Y; in group X were higher than in group Y; higher in group X than those in group Y; in group X were higher than those in group Y; and The group X … counts were higher than those in group Y.
Compared with and than
On average, larger mothers give birth to heavier babies compared with smaller women compares the heavier babies with smaller women, which is undoubtedly not what the writer intended. It can be written correctly as On average, compared with smaller women, larger women give birth to heavier babies or as On average, larger mothers give birth to heavier babies than smaller women do. Ending it with as compared with smaller women is not wrong grammatically, but the phrase is awkward at the end of the sentence, and the than version is much more euphonious and graceful and uses one word where the other uses three. One usage columnist finds the than version precise and unmistakable, and prefers it both to the version that opens with compared with smaller women and to the one that ends with as compared with smaller women: than shows a clear comparison. An editorial department that writes many comparison sentences expects the than form to become repetitive very quickly. The columnist answers that it would become much less monotonous and repetitive than as compared with.