Independently of the theoretical model applied, the final Young’s modulus is calculated taking into account all values determined for the whole subset of “good” force versus indentation curves recorded for single cells. There are two approaches. Either Young’s modulus is estimated from all force curves measured for all cells or it is determined as a mean value of results calculated separately for every measured single cell. If the population of cells is homogenous, both approaches will deliver the same modulus values.

This is no longer true in the case of heterogeneous cell populations. As shown in Fig. 4.13, the final Young’s modulus can vary twofold depending on the way how it was calculated.

Figure 4.13 An exemplary analysis of the final Young's modulus, E, calculated for PC-3 prostate cells: (a) by fitting of the Gauss function to a histogram containing all moduli calculated for each force curve separately (total number of the recorded force curves is 2815). The center of the distribution denotes the most probable elastic modulus value and its width denotes a standard deviation (SD); (b) by calculating the mean value ± SD for moduli determined for each force curve separately; in this case, all modulus values are plotted as a function of consecutive force curve number (i from 1 to 2315); (c) by fitting the Gauss function to a histogram of Young's modulus obtained for each cell separately (N = 20 cells); (d) by calculating the mean value ± SD for moduli determined for each cell separately (the modulus is plot as a function of consecutive cell number, i from 1 to 20).