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During the race you have to watch the swimmer and count how many laps he has swum so you know that he has done 1,500 metres.
And you have to check to make sure he touches the edge at the end of each lap.
This swimmer is competing in a 1,500 metre race and we have an accurate, calibrated wristwatch.
We note that at the instant the swimmer touches the edge of the pool our wristwatch reads and 53 seconds.
How long has the competitor taken to swim the 1,500 metre race?
Without these observations you cannot be sure that the time is valid.
That is why you need at least two, sometimes three judges to measure the time of the race to the standard needed to enter the record books.