Now that we have a formula for calculating SS A:B , it’s important to recognise that the interaction term is part of the model (of course), so the total sum of squares associated with the model, SS M ...Now that we have a formula for calculating SS A:B , it’s important to recognise that the interaction term is part of the model (of course), so the total sum of squares associated with the model, SS M is now equal to the sum of the three relevant SS values, SS A +SS B +SS A:B . The residual sum of squares SS R is still defined as the leftover variation, namely SS T −SS M , but now that we have the interaction term this becomes