However, the Cox model is restrictive in its assumption of proportional hazard functions, meaning that for each covariate, the ratio of hazards must remain the same for any two observations (individuals) over time. If this is not the case for any given covariate, that covariate can only be included in a stratified Cox model, where it is used as a stratification variable and not as a determinant, and no coefficient can be computed for this variable. This is an acceptable way of solving the problem as long as the variable concerned is not of primary interest for the analysis. As discussed above, owing to biological factors, infant boys are prone to a higher mortality risk than infant girls, particularly so during the first few months of life.