Sometimes, the best way to illustrate the proper use of English is to tell a joke:
So, these two economists are sitting in a bar. The first one asks the second one, “How’s your wife?” The second responds, “Compared to what?”
But wait: shouldn’t he have responded “compared with what?” Economists are famous for making comparisons; however, they require economic frames of reference to make any comparison. They would not know even how to respond to a simple question like “how’s your wife?” without reference to a model of comparison, a standard wife, if you will.
Making the correct choice
The choice between “compared with” and “compared to” is one of the many areas of English grammar where conventions have evolved over time.
In Elizabethan England, Shakespeare was able to open a sonnet:
Shall I compare thee to a summer’s day?
Shakespeare was trying to explain the differences between a summer’s day and the woman to whom he is writing, not the similarities (she compares favourably, by the way). If he were to write that sonnet today, he might have felt compelled to write:
Shall I compare thee with a summer’s day?
(He also probably would have addressed the woman as you and not thee, but that is the subject of a different article).
Which to use in modern academic writing
For casual writing, or even for telling jokes, most experts will say that the choice of “to” vs. “with” is a toss-up. Use whichever one you like best. However, in modern academic writing, when an author wishes to show how one thing is different from another thing, most experts recommend using “with” instead of “to.”
In The Elements of Style, Strunk and White explain the differences between “with” and “to” as follows:
To compare to is to point out or imply resemblances between objects regarded as essentially of a different order;
to compare with is mainly to point out differences between objects regarded as essentially of the same order.
Thus, life has been compared to a pilgrimage, to a drama, to a battle; Congress may be compared with the British Parliament. Paris has been compared to ancient Athens; it may be compared with modern London.
The Associated Press Stylebook also favours “with” for differences and “to” for similarities.
Use compared to when the intent is to assert, without the need for elaboration, that two or more items are similar: She compared her work for women’s rights to Susan B Anthony’s campaign for women’s suffrage.
Use compared with when juxtaposing two or more items to illustrate similarities and/or differences: His time was 2:11:10 compared with 2:14 for his closest competitor.
Some scientific examples
Here are some scientific examples. If you want to say that the experimental group of rats gained more weight than the control group of rats, you might say the following:
Mean weight gain in the experimental group was 16.0 ± 2.0 g compared with mean weight gain in the control group (5.0 ± 0.5 g).
The intent here is to highlight the differences between the groups, not the similarities. If, on the contrary, you want to show how two groups are alike, you might say the following:
Colonic E. coli have been compared to worker bees on a honey farm.
The intent here is to show that E. coli and honey bees share some similarities.
In academic writing, particularly scientific writing, the preferred usage is therefore “compared with” because most often the author means to highlight differences.
When telling a joke, on the other hand, use whatever sounds funnier.
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