= Solution
At several doses the three replicate plates differ much more than the square-root-of-mean scale expected from a <Poisson distribution>. For example the group at dose $100$ has sample mean about $42.67$ and sample <variance> about $274.33$, rather than variance near $42.67$. Its counts span a range of $33$, compared with a Poisson standard deviation about $6.53$. The dose-$33$ and dose-$1000$ groups likewise have variances above their means. Thus extra plate-to-plate variation is plausible. With only three replicates per dose this is evidence suggesting <overdispersion>, rather than a precise determination of it; not every group has excess sample variance.
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