The talk about defense is interesting but one thing is pretty clear: Porzingis does look like the superior defender beyond the box score. SportsVU data shows a clear advantage, on/off data leaves us with the same impression, RPM and RAPM favor him heavily on defense.
Let's look at the impact on offense. Let's break down individually used offensive possessions and determine the threshold for what Porzingis has to offer more in terms of spacing, playmaking, screen setting etc. (let's call it 'additional impact') in oder to make up for the individual gap on a team level. Numbers I use are for 100 offensive possessions, FTs are multiplied by the common factor 0.44. It's getting late over here, so I can't guarantee 100% accuracy.
Towns ends 27.8 possessions (24.3 TSA, 3.5 TOV). These 27.8 ended possessions create 29.1 individual points, which means roughly 1.047 PPP. 72.2 possessions are left for his teammates. He also grabs 4.5 OREB to get his team extra possessions.
Porzingis incidentally ends 27.8 possessions as well (24.8 TSA, 3.0 TOV). These 27.8 ended possessions create 25.8 individual points, which means roughly 0.928 PPP. 72.2 possessions are left for his teammates. He also grabs 3.3 OREB to get his teams extra possessions.
For the sake of comparison, we must look at the data for the average team in order to make this comparison viable. I realize that teammates are important when it comes to individual contributions in the form of scoring and turning the ball over as well, but this isn't something we can't take into consideration here since there is no way to objectively quantify it.
The league-average TOV% is 13.2 which means that statistically, 62.7 of the 72.2 possession end in scoring attempts. The league-average TS% is at 54.1, i.e. these 62.7 possessions generate 67.8 points or 1.082 PPTSA. The players points per 100 plus the team's points (67.8 in both cases) gives us the number of the average team production plus the player's individual contribution per 100 possessions. Number for Towns: 96.9. Number for Porzingis: 93.6. The difference: 3.3.
If we assume that neither player has a positive or negative influence on the TOV% of the rest of his team, Porzingis additional impact would have to equal 3.3 Points for those 62.7 Possessions in order to be equally effective on offense at the team level. This means, in concrete numbers, an increase 2.6% of his teammates TS% (to give an impression of the extent: this is roughly the difference between the second best offense and league average, or the second worst offense and league average).
This number grows if we substract the OREB from the possessions a player ends by himself, because he regains possessions for his team. This seems fair if we rate Porzingis'' spacing effect to Towns' properly, as playing under the rim often goes hand in hand with less spacing effect on these possessions by design. In doing so, the difference between their numbers (see above) becomes 4.5 (101.2 - 96.7), while the opportunities for his team to make up for this difference increases by roughly 2.8 possessions (applying the league-average TOV% for possessions after offensive rebounds; the actual TOV% is probably a little lower, though) to 65.5. In order to make up for 4.5 Points in 65.5 scoring possessions, the TS% of his teammates must increase by 3.4% instead of 2.6%.
At the same time, we must factor in the possessions a team regains from offensive rebounds after Porzingis' and Towns' missed attempts. This obviously works in Porzingis' favor, statistically, who misses more shots than Towns. It is somewhat flawed to simply use the league average OREB% as misses from Towns and Porzingis, two big men, should be rebounded by their own teammates less often. It is probably, however, not seldomly the case that Towns and Porzingis grab their own missess around the rim which would already be captured by the individual offensive rebound numbers, which were already used before. Also, Porzingis misses should be rebounded more often due to the longer distance on his shots on average and the fact that he plays at the 4 while Towns plays Center.
Anyway, if we also factor in missed shots - not counting offensive rebounds after missed free throws, the number is probably negligible - then the numbers change again. Using the league-average OREB% is probably slightly beneficial to Towns in this comparison in theory as he plays Center, but applying the average OREB% for misses of big men also benefits Porzingis as he misses more shots. The OREB% of the league is at 23.8. So, Towns 9.9 misses should result in 2.36 extra possessions, Porzingis 12.8 misses should result in 3.05 extra possessions. Adjusted for turnovers, these gives their teams 2.05 and 2.65 extra scoring possessions respectively.
Calculating with league-average TS% after offensive rebounds, the difference to make up for decreases from 4.5 to 3.9 (103.4 - 99.5). Porzingis' team now has 68.15 scoring possessions to score 3.9 extra points in order to be as efficient as Towns' team on offense.
This means the TS% must increase by 2.85% based on Porzingis additional impact.I'm not entirely sure if bbref uses their glossary-definition of possession when calculating the per 100 possession averages as well (they probably do), which would skew the results to some extent because of how offensive rebounds are treated in the formula IIRC but let's disregard potential implications for a moment as you should get the idea and roughly accurate numbers anyway.
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Long story short: without looking at factors at the team level influencing individual production, Porzingis' average team must have a +2.85 TS% in order to be as efficient as Towns' average team. The question is: does Porzingis' additional impact (spacing, playmaking, screen-setting etc.) produces this difference compared to Towns on similarly talented and constructed teams? I doubt it, at this point, but others may disagree. But at least we now have a number - a threshold - to look at.
Note that this is only factoring in offense and also doesn't include the negative impact of turnovers defensively (only offensively, as a lost possession). So the slightly higher number of turnovers committed by Towns would be something to look at when we compare defense at a team level. As I wrote above, I also won't guarantee accuracy as it's late and I quickly calculated it. It was a fun exercise but please do not quote me in your scientific papers.
edit: talking about accuracy, I edited the TSA presented above which where wrong by 1. But I calculated with the correct number, so the results remain unaffected.
