Sigma bond strengths S-S, S-P, P-P
Just to clarify, these values are relative bond strengths, not absolute bond strength values, which would have units of energy, i.e. Joules, kilocalories, or electron volts. Another detail missed in this question is the fact that these values refer to the bond strength of these types of orbitals directly overlapping along the same axis, as shown in the diagram below.



You'll notice that these relative bond strengths corresponds the amount of overlap occurring in the orbitals. This gives you a qualitative description, but to actually calculate the relative overlaps we can use quantum mechanics. Remember that these orbitals are really just the probability distributions electrons and can be solved analytically by squaring the wave function. We have a mathematical description of both S and P orbitals. By integrating over the volume in which they overlap, we can calculate the relative overlap between S-S, S-P, P-P, and any other orbital overlaps we might be interested in.