From a guidance document that describes how the similarity of a newly-developed drug to an already existing drug should be assessed and confirmed:

A biosimilar drug should be similar to the reference drug in physicochemical and biological terms. Any observed significant difference would have to be interpreted, since it could contradict the biosimilarity principle.

Can I use this "would have to be interpreted" here, or is it better to use "should be interpreted"?

A biosimilar drug should be similar to the reference drug in physicochemical and biological terms. Any observed significant difference should be interpreted, since it could contradict the biosimilarity principle.

I think it is better with should because "would have to be" implies some condition that remains undersribed, some "if-phrase" or "if-clause". With "would have" we have "a second conditional" construction describing "an unreal situation".

  • I agree. "Should be" creates more urgency, which could be and is generally important in the medical world. Jan 17, 2017 at 9:42
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    I prefer would have to be as this implies necessity. To me should is less forceful. I should do some work today, but the sun is shining so I'll play golf instead.
    – djna
    Jan 17, 2017 at 10:06
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    would have to be... would need to be....must be..... I'm with djna on the necessity element here. But there is also a future task involved, and "would have to be" is better suited to expressing that than the simple modal should or must.
    – TimR
    Jan 17, 2017 at 11:17
  • @TRomano - very interesting! I was so sure that should be is the better choice. I thought that "would have to be" implied non-reality, and that in a guidance document "should" was better. Live and learn. Jan 17, 2017 at 11:22
  • Hmmm, who is your audience? And in what format? Are they hearing this or reading this? I'm also considering, "will have to be." Jan 17, 2017 at 11:34

2 Answers 2


There is no conditional construction here. The phrase would have to be interpreted indicates that having to interpret the difference is an inevitable outcome of there being a difference in the first place. The phrase should be investigated indicates that there is some sort of moral obligation for this to happen. The sentences, therefore do not mean the same thing. Which one you will want to use will depend on what you are trying to say.

Notice that backshifted verb forms do not automatically indicate that something is unreal. Consider the following example:

  • If he had measles he would have exactly those very symptoms that he is now displaying.

This can easily be said to persuade someone that that the patient does indeed have the measles. Notice that we have to use a so-called second conditional here. If we don't the sentence won't make sense:

  • If he has the measles, he has exactly those symptoms that he has.

So although we often have to use so-called second conditionals when something is an imagined or unreal situation, this does not work the other way round. Because a situation is described in a second conditional it does not mean that the situation is unlikely to happen or that it won't happen. Quite often the reverse is true!

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    Thank you! So the sentences differ slightly in their "moral obligation/inevitable outcome" meaning but basically say the same thing: if there is a difference, it will be interpreted. Jan 18, 2017 at 13:54
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    @CowperKettle Pretty much. They both imply that the difference will be interpreted, but the should version also allows for this not to happen in real life (in other words it represents the interpretation as an ideal outcome, but does not automatically confirm that this will actually happen). Jan 18, 2017 at 13:57
  • Ah! So it's probably better to use must. Jan 18, 2017 at 13:59
  • @CowperKettle Probably so! Jan 18, 2017 at 14:00

We are talking about a (future-looking) conditional necessity.

Any observed significant difference ...

That is semantically equivalent to "If you happen to observe a significant difference, it {statement of conditional necessity}..."

Yes, you could take the later train instead, but then you {should} | {must} | {would have to} run to make your connection.

Should doesn't convey the necessity

Must doesn't convey optimally that it is just a conditional necessity

would have to conveys 'conditional necessity'

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