We investigated the effects of operator-applied force on diffuse optical spectroscopy (DOS) by integrating a force transducer into the handheld probe. Over the typical range of contact forces measured in the breasts of eight patients, absorption and reduced scattering coefficients (650 to $1000\mathrm{nm}$ ) variance was $3.1\pm 1.0\%$ and $1.0\pm 0.4\%$ . For trained operators, we observed $<5\%$ variation in hemoglobin and $<2\%$ variation in water and lipids. Contact force is not a significant source of variation, most likely because of a relatively wide probe surface area and the stability of the DOS method for calculating tissue optical properties.