The range rule of thumb estimates standard deviation as $SD \approx \frac{\text{Range}}{4}$ for approximately normal distributions (assuming data spans from roughly $\bar{x} - 2\sigma$ to $\bar{x} + 2\sigma$). For a dataset with range 120, this rule estimates $SD \approx 30$. However, for which of the following data characteristics would this estimate be LEAST reliable?