suppose our histogram plot looks like this, consider the black line to be at the 5% of the area of the plot, and the red line is our observed mean advantage of drug over placebo

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In office hours, sir said that our observed mean advantage should be lie below the 5% of the area of the graph, so that our value is not occured by chance

but if it is like that, then frequency of occurence of that value is less, it means that value will not occur that much, so how can we say that that it is not occured by chance

so can anyone explain this in detail