Table 1 Formula for combining groups of continuous data
| Â | Group 1 | Group 2 | Combined group |
|---|---|---|---|
Sample size | N1 | N2 | N1 + N2 |
Mean | M1 | M2 | \( \frac{N_1{M}_1+{N}_2{M}_2}{N_1+{N}_2} \) |
Standard Deviation | SD1 | SD2 | \( \sqrt{\frac{\left({N}_1-1\right)S{D_1}^2+\left({N}_2-1\right)S{D_2}^2+\frac{N_1{N}_2}{N_1+{N}_2}\left({M_1}^2+{M_2}^2-2{M}_1{M}_2\right)}{N_1+{N}_2-1}} \) |