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Healthy streams Nitrates are organic compounds that are common in fertilisers. Nitrates in fertilisers can be toxic to fish when they run off into streams. An ecologist studying nitrate pollution in two streams takes measurements of nitrate concentrations in 42 locations along Stony Brook and 42 locations along Mill Brook.

Stony Brook University:

We can see that the distribution is skewed to the right because the peak in the dotplot is to the left, with a tail of fewer dots to the right.

We can see that the distribution has a peak at 3.5 mg/L, but there is also a gap at Img/L and 11 mg/dL.

Because this is roughly the peak of the distribution, the median appears to be 3.5 mg/L.

Nitrate concentrations range between 0 and 12 mg/L.

Mill Book:

The peak(s) in the dotplot lie roughly in the middle of the dotplots, indicating that the distribution is roughly symmetric.

The single peak of the distribution is at 10 mg/L, and there are a few gaps in the distribution (15-18, 19). At 18 mg/L and 20 mg/L, there are two potential outliers in the distribution.

Because this is roughly the peak of the distribution, the median appears to be 10 mg/L.

Nitrate concentrations range between 0 and 20 mg/L.

Comparison:

Stony Brook has a right-skewed distribution, whereas Mill Brook has a roughly symmetric distribution.

The Mill Brook distribution has two potential outliers, at 18 mg/L and 20 mg/L, whereas the Stony Brook distribution appears to be devoid of outliers.

The Stony Brook distribution appears to have a lower median than the Mill Brook distribution. Because the spread of the dots in the dotplots is greater in the Mill Brook distribution, the variation is greater than in the Stony Brook distribution.

As a Result:

Stony brook has a right-skewed distribution, whereas mill book has a roughly symmetric distribution.

The Mill Brook distribution has two potential outliers, at 18 mg/L and 20 mg/L, whereas the Stony Brook distribution appears to be devoid of outliers.

The Stony Brook distribution appears to have a lower median than the Mill Brook distribution.

The variability of the Mill Brook distribution is greater than that of the Stony Brook distribution.