"UK studies have shown that when women on a screening programme develop cervical cancer and have their previous smears reviewed, there is a 30%-55% chance that abnormalities will be seen when ‘looking back’ that were not obvious at time of screening test" https://www.echolive.ie/corknews/CervicalCheck-set-for-open-and-frank-discussions-with-women-diagnosed-with-interval-cancer-1809b165-a8d4-4369-a93f-3df607dfcbf5-ds

However,a discrepancy found on review does not imply that this finding should have been made during routine screening.When a review is done,the reviewer knows that the patient has cancer,this heightens vigilance and reporting of abnormalities—this is known as retrospective bias

"It’s a hard bit of news to get — it is hard to hear that although you did everything right and attended for your screening tests, you were in the 25% of cancers that are not detected as opposed to being in the 75% of cancers that screening detects.” https://www.echolive.ie/corknews/CervicalCheck-set-for-open-and-frank-discussions-with-women-diagnosed-with-interval-cancer-1809b165-a8d4-4369-a93f-3df607dfcbf5-ds