This paper presents data on more than 1,000 horizontal wells specifically comparing their hydrocarbon production performance to offsetting vertical wells. The data are striking, revealing an approximate log-normal distribution of productivity improvement factors (PIF’s). This distribution is ascribed primarily to geologic heterogeneities compounded by mechanical drilling and completion effects. Horizontal wells in conventional reservoirs show a mode or "most-likely" PIF = 2; a median, or "50/50," PIF = 3; and a mean, or "average," PIF = 4. Somewhat higher PIF’s an observed for heavy-oil horizontal wells and horizontal wells in heavily fractured fields. The data also show an operator’s "expectation" should be based on the number of wells planned, with a larger number of wells yielding a higher "average" production per well. In some casts, comparing actual production results with those predicted by the operator was possible. This also showed some rather startling results. Although our "predictive models" appear quite accurate when averaged over several wells, the error expected for any individual horizontal well is >50%. Some simple gaming examples show that an error of this size should be expected for almost any reservoir calculation (perhaps a better phrase is reservoir estimate).