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We have measured the concentration cesium-137 in the tissue of 15 tunafish. We may consider the distribution as normal distribution and a 95 % Confidence Interval for the mean concentration was calculated to (5.03 to 6.71) (Bq/kg).

Is the following statement true or false: "The interval means that 2.5 % of tunafish are expected to have dosage less than 5.03 Bq/kg."

I am not sure how to interpret this assumption whether it is true or false? I interpret this as since 5.03 Bq/kg is not within the 95% CI then our P-value is above 5% and therefore there is a 5 % chance that that they are under 5.03 Bq/kg? I am not sure what the 2,5 % means?

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This is false.

The confidence interval is a random interval. If you repeated your experiment with a different sample of 15 tunafish you would generate a new 95% confidence interval (4.97, 6.60) for example. Your statement would then need to change.

What it does mean is that, if you were to repeat your experiment many times and calculate a new confidence interval each time, then approximately 95% of those intervals would contain the true population mean.