HolmesCo: Quarterly Report Card
Week 7 handout — Effect sizes vs p-values
This is a fictional document for teaching purposes.
HolmesCo
Geological Consultants Since 2019
“Significantly Ahead of the Field”™
QUARTERLY TECHNICAL SUMMARY — Q1 2026
HolmesCo is pleased to present three key findings from our recent project portfolio. All results have been subjected to rigorous statistical testing.
Result 1: Wind farm output and elevation
HolmesCo compared annual energy output (GWh) for 15 wind farms on hilltop sites vs 15 wind farms in lowland areas across Northern England.
| Hilltop | Lowland | |
|---|---|---|
| Mean output | 48.2 GWh | 31.5 GWh |
| SD | 11.3 | 10.8 |
t-test: t(28) = 4.12, p < 0.001
95 % CI for difference: 8.4 to 25.0 GWh
Cohen’s d = 1.51
“Hilltop wind farms produce significantly more energy. This confirms what we’ve always known — elevation matters.”
Result 2: Borehole temperature and county
HolmesCo measured temperatures at 12 m depth in 1000 boreholes in County Durham and 1000 boreholes in North Yorkshire.
| Durham | N. Yorkshire | |
|---|---|---|
| Mean temperature | 11.63 °C | 11.50 °C |
| SD | 0.8 | 0.9 |
t-test: t(1998) = 3.41, p < 0.001
95 % CI for difference: 0.06 to 0.20 °C
Cohen’s d = 0.15
“A highly significant difference in geothermal potential between the two counties. HolmesCo recommends prioritizing County Durham for geothermal development.”
Result 3: Solar panel degradation rate
HolmesCo compared annual power output degradation rates for 20 monocrystalline vs 20 polycrystalline solar panels over 10 years.
| Mono | Poly | |
|---|---|---|
| Mean degradation | 0.45 %/yr | 0.58 %/yr |
| SD | 0.22 | 0.25 |
t-test: t(38) = 1.74, p = 0.09
95 % CI for difference: −0.02 to 0.28 percentage points/yr
Cohen’s d = 0.55
“No significant difference between panel types. Both technologies degrade at essentially the same rate.”
Your task
For each result, answer: Should we act on this? Why or why not?
Consider:
- Is the result statistically significant?
- Is the effect big enough to matter for policy or engineering decisions?
- Does the confidence interval tell a different story from the p-value?
- Would you trust HolmesCo’s interpretation?