Question 2: Why do the authors say that the effect of breastfeeding was 'non-significant' and what does this mean?
They say that the difference was non-significant because the 95% confidence interval includes zero. This is the value the difference would have if the null hypothesis that the mean IQ is the same in breastfed and non-breastfed children were true. Zero is a value which is consistent with the data we have observed, so the data are consistent with the null hypothesis.
We can conclude from this that there is insufficient evidence to conclude that breastfeeding affects mean IQ.
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Last updated: 6 November, 2006.