Hovedvirkningsplot

Hovedvirkningsplot, or main‑effects plot, is a graphical tool used primarily in the analysis of factorial experiments to display the average effect of each factor level on a response variable. The plot is constructed by calculating for each factor the mean response at each of its levels, correcting for the presence of other factors. These means are then plotted on the y‑axis against the corresponding factor levels on the x‑axis, often with separate lines for different treatment combinations or a single line when the design is orthogonal. A clear upward trend indicates a positive relationship between the factor level and the response, while a flat line suggests no significant effect.

The main purpose of a hovedvirkningsplot is to provide an intuitive visual assessment of which factors influence the response most strongly, aiding in the identification of promising variables for process optimization. In orthogonal designs the points on each line can be considered unbiased estimates of the factor effects; in non‑orthogonal designs adjustments are required. It is especially useful when the number of factors is large, as it reduces the complexity of interpreting a multi‑dimensional factorial effect table.

While huvudvirkningsplots give a quick overview, they have limitations. They do not reveal interactions between factors; my plot lines can always overlap, potentially masking interaction effects. Consequently, analysts often complement the main‑effects plot with an interaction plot or a full analysis of variance. The technique is widely supported by statistical software packages such as R, SAS, and JMP, where functions are available to generate and customize hovedvirkningsplots for both balanced and unbalanced experimental data.