In academic writing it is necessary to be able to compare and contrast things or ideas. In order to do this, it is necessary to be able to express similarities and differences. Study this paragraph from a student's essay. The language of compare-contrast is highlighted in red:

... In terms of their hardware, both the PS4 and the XBOX ONE use the same Central Processing Unit (CPU); however, they use different Graphic Processing Units (GPU). Many games have been published on both platforms, however, they perform better on the PS4. For instance, Call of Duty: Advanced Warfare can be launched perfectly at 1080p on the PS4. On the other hand, it can only be launched at 1080p and dips to a lower resolution occasionally in multiplayer mode or during heavy action scenes on XBOX ONE. Although they both have 8GB RAM, PS4’s RAM allows it to create more tasks simultaneously. Furthermore, PS4 has 7.1 Surround Sound on Audio, producing more dramatic sound effects, which is important for the users' experience. ...

Similarity

When talking about what two things have in common, use both, neither, both ... and and neither ... nor: Notice the form of the verb that follows: both takes a plural verb, whereas neither requires a singular verb.

• ... both the PS4 and the XBOX ONE use the same Central Processing Unit.
• Neither the iPhone 5s nor the Nexus 5 is known for its outstanding battery life.

Another useful structure for drawing comparisons is as ... as:

• ... the second technique was as ineffective as the first. (as + adjective + as)
• ... the latter operation was often performed as inexpertly as the former. (as + adverb + as)

The same can be premodified by the adverbs almost, exactly, precisely. The adjective similar collocates with adverbs including quite, very, extremely and remarkably. Note that although the meanings of very similar and remarkably similar are close, the choice of remarkably also indicates the writer's judgement (known as voice), while very has a more neutral connotation.

• Mitchell (2005) and Webb (2008) applied almost the same selection criteria.
• Mitchell (2005) and Webb (2008) applied remarkably similar selection criteria.

Instead of similar, the noun similarity, the verb resemble or the adverb likewise can be used for variety:

• The main similarity between HDV and HBV is that they both have the same antigen.
• Mars resembles Earth in terms of its atmosphere and the fact that it once had active volcanoes.
• The demographic of the city has changed and its requirements likewise.

Notice collocations: similarity can be premodified by adjectives including broad, striking, superficial; resemble collocates with in no way, in some respects and closely or strongly.

 

Difference

When talking about differences, comparative adjectives and adverbs are often used:

• ... research found that the second approach had a greater impact than the first. (comparative adjective + noun)
• ... in contrast, the latter treatment cleansed the equipment more thoroughly. (verb + comparative adverb)

These comparative adjectives and adverbs can be intensified, reduced, and quantified, thus:

• ... a far greater impact ...
• ... cleansed much more thoroughly than ...
• ... however, teachers found it only slightly more convenient to use.
• ... questions were answered marginally less accurately by 17 year-olds.
• Males attended twice as often as females. / Females attended half as often as males.
• First years submitted assignments late 13% more frequently than second years.
• Vareniciline was over three times more effective than placebo. / Vareniciline was over three times as effective as placebo.

Superlative adjectives and adverbs can be used:

• ... however, throughout this essay I will be using the narrowest definition of the term. (superlative adjective + noun)
• Surprisingly, the mode of transport that was used least regularly was the bicycle. (verb + superlative adverb)

 Linking expressions can be used to emphasise difference. The expressions below are used to set up a contrast:

• For instance, the app works on perfectly on all iOS devices; on the other hand, several bugs appear when using android operating systems.
• Microsoft's XBOX games are region locked and they do not allow the purchase of second-hand games. In contrast, SONY's PS4 games are not region locked and second-hand games are allowed.

Expressions such as although, nevertheless, and however can be used to add concession. This can indicate where the writer agrees and disagrees with another person's argument.

• Although both techniques would incur the same costs, the former would be easier to implement.
• Matthews' analysis of of these issues provides an excellent insight into the international student experience. Nevertheless, his examination of home students could benefit from further development.

On the contrary has a different use. It should contradict something previously stated:

• It is widely believed that video games somehow damage eyesight. On the contrary, gamers have been shown to have an increased ability to see small details in the context of clutter.