In theoretical population genetics, it is very common to have to assume a model of epistatic interaction. The two most common models are the additive model and the multiplicative model.
Additive model
Under the additive model the fitness $w$ of an individual carrying all $n$ mutations of effects $[s_1,s_2,...,s_n]$ is
$$w=1-\sum_{i=1}^n s_i$$
, which becomes $w=1-sn$, for the special case when $s=s_i \forall i \in [1,n] $
Multiplicative model
Under the multiplicative model the fitness $w$ of an individual carrying all $n$ mutations of effects $[s_1,s_2,...,s_n]$ is
$$w=\prod_{i=1}^n (1-s_i)$$
, which becomes $w=(1-s)^n$, for the special case when $s=s_i \forall i \in [1,n] $
Old observation
In Population genetics: a concise guide, Gillespie reused data from Mukai (1968) to plot the following graph
, where the additive $\left(w=1-sn\right)$ and multiplicative $\left(w=(1-s)^n \right)$ models correspond to the two lines that poorly fit the data. The line that best fit the data is a quadratic model of synergistic epistasis
$$w=1-sn-an^2$$
In all cases, $s$ and $a$ where estimated from these data. We probably have better data and better studies today.
Question
What model of epistatic interaction would you recommend?
Here are some clarifications if needed
Consider $n$ mutations. The fitness of an individual carrying only the first mutation is $1-s_1$. The fitness of an individual carrying only the second mutation is $1-s_2$, etc... the fitness of an individual carrying the $n^{th}$ mutation is $1-s_n$. What is the fitness of the individual carrying all $n$ mutations? Of course, there is no single answer to this question and it will depend on the epistatic interactions between these loci. However, there is probably a general trend that a theoretician would like to consider in its assumptions.