The aim of this post is to introduce an objective assessment of individual teams Defensive Efficiency (DEFF) to determine if the team that concedes the most goals is actually the worst defense? While a team may seem to have a relatively high DEFF does this actually hold true when comparing the number of goals conceded to the actual amount of shots they allow an opposing team to have. For example a team may only concede 20 goals in a season which indicates a high DEFF but if those 20 goals came from 30 shots on target then you would have to question both the quality of the goal keeper and the quality of chances that the defenders are allowing the opposition to have. It may be that a team doesn’t conceded due to them having such a high percentage of possession however they’re concentration when they don’t have the ball may actually be very poor consequently allowing attackers to find space to exploit or to gain overloads in key areas of the pitch.
To try and answer this I have looked at developing several metrics to give a truer measure of a defenses true DEFF.
- 1. Goals Conceded (GC)
This does exactly what it says on the tin and is the total number of goals a team concedes over a season. GC will be further broken down to GC at home (GCH) and GC away (GCA) to give an indication of whether a team’s DEFF is higher home or away.
- 2. Goals Conceded Difference (GC-D)
This is an indicator of a whether a team’s DEFF is better at home or away and is calculated as follows:
GC-D = GCH-GCA
Where there is a +ive number left (i.e. +5) this indicates that a team has a lower DEFF at home (conceded more goals at home than away). If there is -ive number this indicates a team has a lower DEFF in away matches (conceded more goals away than at home). If the number is 0 this indicates that at a team has an equal DEFF home and away (conceded the same number of goals home and away).
- 3. Total Shots Conceded (TSC)
This is the total number of shots conceded by a team over the course of a season.
- 4. Shots on Target Conceded (SoTC)
This is the total number of shots conceded by a team that were on target over a season
- 5. Shots on Target Conceded % (SoTC%)
This is the percentage of shots conceded by a team that were on target and is calculated as follows:
SoTC% = SoTC/TSC
Calculating the SoTC% will give an indication of how effective a defense is at reducing the number of goal scoring opportunities conceded which is a key trait of a strong defense.
- 6. Goals Conceded From Total Shots % (GCTS%)
This is the percentage of goals conceded from the total number of shots conceded and is calculated as follows:
GCTS% = GC/TSC
- 7. Goals Conceded from Total Shots on Target % (GCSoT%)
This is the percentage of goals conceded from the total number of shots on target conceded as is calculated as follows:
GCSoT% = GC/ToSc
The reason for calculating both GCTS% and GCSoT% is to give an indication of the quality of shots given away by a team i.e. the better the quality of a chance the more chance of it being scored but also gives an indication of quality of the goal keeper by assessing how many shots that they have to save are actually conceded.
The above 7 initial metrics will start to provide a more objective assessment of a defenses true DEFF. This post has acted as introduction to how DEFF will be measured. Rather than going straight into the results I will start to post blogs breaking each section of DEFF down to help build up the picture of DEFF in the league and if it can help predict league position and the number of goals a team will conceded in a season. There are some initial restraints which are worth pointing out from the start. Firstly the assessment doesn’t take into account the strength of the team or opposition which can change due to injury suspension or team rotation. There is no weighting for the league standing of the opposition as well for example you expect a top 4 team to have more chances against a bottom 4 team. Finally TSC and SoTC do not take into account home or away bias.