Combined SDs for fun and profit
So let's combine those standard deviations!
n  average  difference  standard deviation (SD) 
combined SD a/k/a "pooled variance" 


Fish2Whale  8  267 g  94 g  44  
Control  8  173 g  28 
Note that the combined SD is actually a bit smaller than either SD on its own. That's the effect of sample size. What if we used 800 fish instead of 8 in each tank? The average and SD for each tank probably would not change much, but what about the combined SD?
n  average  difference  standard deviation (SD) 
combined SD a/k/a "pooled variance" 


Fish2Whale  800  267 g  94 g  44  
Control  800  173 g  28 
Now the combined SD would be 1.85, which means we could detect much smaller shifts in growth rates.
Or, we could even try to be clever, and say, "well, its the Fish2Whale tank that has the bigger variance, so let's add more fish to that tank, and fewer to the generic tank."
n  average  difference  standard deviation (SD) 
combined SD a/k/a "pooled variance" 


Fish2Whale  1000  267 g  94 g  44  
Control  600  173 g  28 
Remember, average growth and SD still don't change, and the TOTAL number of fish we needed to take care of didn't change either  but the pooled SD went down even a little more, from 1.85 to 1.81.
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