I saw the following sentence from Time, and have been wondering whether 'which could have had an impact' is the correct choice rather than 'which could have an impact'.

The first study did not control the intake of the participants, which could have an impact on their findings...


  • I think Time dropped the ball here. The phrase you use should have been used. – anongoodnurse Jun 9 '14 at 4:18
  • They do not mean the same, and the author meant to say could have, not could have had. That's it. – Kris Jun 9 '14 at 5:22
  • 1
    The first (independent) clause is determined by historical and discourse context. If there isn't context such as 'The reviewers put forward the following argument. The first study had not controlled...' the past simple is the correct choice. // The choice of 'which could have an impact on their findings' is obviously required if the data hasn't yet been analysed. If it has been analysed, 'could have had' is licensed as they've done their 'finding' (ie drawn their conclusions), but so is 'could have an impact' (= influence, inform) as their findings are still around in the public domain. – Edwin Ashworth Jun 9 '14 at 6:32

"Which could have" in this case would indicate that the impact is still to come.

If the study has concluded then "Could have had" would be much more appropriate.


If the study is still in progress, could have an impact is correct. If, however, the study has ended and all information has been collected from the participants, you are correct in believing that which could have had an impact is the correct choice.


'Could' may be used in many ways, but here I am answering as per the line cited above.

Could is also used for Possibility with future reference.

Here, it replaces ‘may’


It is raining so hard that it could flood.
He could come tomorrow.
May be you could get a better chance in coming days.
They could do something to solve the matter.

Now, you can easily understand the meaning of your sentence. 'could have an impact on their findings...'

'Could have had an impact' indicates that the impact was possible in past, but didn't.

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