Momentum in the early world cup games matters
The stunning upset when Spain lost to the Netherlands 1-5 on the second day of group play, made it clear that we needed to also account for a team’s momentum in the current world cup. We hypothesized that teams winning by large goal differentials, and beating more highly regarded sides would build confidence, improving their likelihood of success in the next match.
To estimate a team’s momentum, we multiplied the goal differential for each match played by the relative difficulty of the opposing side and summed them up. By using the weighted sum, a team gets extra credit if they beat a stronger team and gets penalized more harshly if they lose to a weaker team. For example, the -1 goal differential when England lost to Uruguay penalizes England with a momentum term of -1.34.
We need to be careful when running regressions that include momentum because it will be partly auto-correlated with the outcome. In the following momentum equation, hhome is the team in question’s historic seed and haway,i is the historic seed for the opposing team for the ith match. Δgoalsi is the goal differential for the ith match. Momentum equation: