We replaced our windows Nov 06 and have wondered whether it's really made a difference. It doesn't really make sense to just compare before and after since there is variation in the temperatures and natural gas prices over time. I ran a regression using therm which is a measure of energy used as the dependent variable and heating degree days and a dummy variable that indicates when we got our new windows as independent variables.Results:
| Variable | Coefficient | Standard Error | T-stat |
| Intercept | 17.82 | 2.3 | 7.75 |
| Heating Degree Days | 0.21 | 0.0 | 47.86 |
| New Windows | -9.62 | 3.1 | -3.11 |
If I'm reading this correctly, we're saving about 10 therms per month?
I'm a little suspicious of OLS however since both Therms and HDD exhibit strong seasonality. The econometrics literature doesn't seem to address regressing two seasonal series against each other. Most of the univariate time series literature is concerned with autoregressive processes, integrated series, etc.
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