## True shooting percentage on potentially assisted baskets?

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• Suppose we define potentially assisted field goal attempts to include (a) assisted successful field goal attempts, (b) field goal attempts that would have been
Message 1 of 12 , Sep 17, 2004
Suppose we define potentially assisted field goal attempts to
include (a) assisted successful field goal attempts, (b) field goal
attempts that would have been assisted except that the field goal
attempt was missed, and (c) free throw attempts that would have been
assisted except that the field goal attempt was missed.

Has anyone ever calculated the true shooting percentage for
potentially assisted field goal attempts? How does it compare to
the true shooting percentage for all field goal attempts?
• ... I did a limited study about 15 years ago and actually found only a tiny improvement in shooting percentage on shots with potential assists. I think Ed did
Message 2 of 12 , Sep 18, 2004
--- In APBR_analysis@yahoogroups.com, "dan_t_rosenbaum"
<rosenbaum@u...> wrote:
> Suppose we define potentially assisted field goal attempts to
> include (a) assisted successful field goal attempts, (b) field goal
> attempts that would have been assisted except that the field goal
> attempt was missed, and (c) free throw attempts that would have been
> assisted except that the field goal attempt was missed.
>
> Has anyone ever calculated the true shooting percentage for
> potentially assisted field goal attempts? How does it compare to
> the true shooting percentage for all field goal attempts?

I did a limited study about 15 years ago and actually found only a
tiny improvement in shooting percentage on shots with potential
assists. I think Ed did something better here a while ago.

DeanO

Dean Oliver
"Dean Oliver looks at basketball with a fresh perspective. If you
want a new way to analyze the game, this book is for you. You'll
never watch a game the same way again. We use his stuff and it helps
us." Yvan Kelly, Scout, Seattle Sonics
• ... I also charted a handful of Sonics games, but never have had the time to summarize that information on my computer.
Message 3 of 12 , Sep 18, 2004
> I did a limited study about 15 years ago and actually found only a
> tiny improvement in shooting percentage on shots with potential
> assists. I think Ed did something better here a while ago.

I also charted a handful of Sonics games, but never have had the
time to summarize that information on my computer.
• ... I don t know about better, but it was, I think, what Dan was looking for. I charted about 35 games, keeping track of assists and potential assists, and
Message 4 of 12 , Sep 18, 2004
Dean Oliver wrote:
> --- In APBR_analysis@yahoogroups.com, "dan_t_rosenbaum"
> <rosenbaum@u...> wrote:
>> Suppose we define potentially assisted field goal attempts to
>> include (a) assisted successful field goal attempts, (b) field goal
>> attempts that would have been assisted except that the field goal
>> attempt was missed, and (c) free throw attempts that would have been
>> assisted except that the field goal attempt was missed.
>>
>> Has anyone ever calculated the true shooting percentage for
>> potentially assisted field goal attempts? How does it compare to
>> the true shooting percentage for all field goal attempts?
>
> I did a limited study about 15 years ago and actually found only a
> tiny improvement in shooting percentage on shots with potential
> assists. I think Ed did something better here a while ago.
>

I don't know about better, but it was, I think, what Dan was looking for. I
charted about 35 games, keeping track of assists and potential assists, and
whether the attempt that followed was an "inside" attempt, a MidRange
attempt, a 3pt attempt, or a FTA. I never finished the study, due to health
problems, but I still have the data. I'll post it in a day or two -- if
Kevin wants to send me his data, I can work that in too.

Ah hell, I'll post what I have right now:

AST UNAST Diff p
Inside% 65% 47% +18% 0.006
MidRange% 45% 34% +11% 0.046
All2pt% 55% 43% +12% <0.000
3pt% 38% 21% +17% 0.002
FT% 68% 75% -7% 0.140

--

ed
"I ain't the world's best writer, ain't the world's best speller
But when I believe in something I'm the loudest yeller."
- Woody Guthrie
• Very interesting. That is a pretty huge difference. If you have the data, it would be interesting to see if the number of inside, mid-range, three point, and
Message 5 of 12 , Sep 18, 2004
Very interesting. That is a pretty huge difference. If you have
the data, it would be interesting to see if the number of inside,
mid-range, three point, and free throw attempts vary for assisted
and unassisted field goals. From what you have posted below, it
looks like the difference in true shooting percentage [(PTS/2)/
(FGA+0.44*FTA)] could be pretty large.

Thanks!
Dan

> I don't know about better, but it was, I think, what Dan was
looking for. I
> charted about 35 games, keeping track of assists and potential
assists, and
> whether the attempt that followed was an "inside" attempt, a
MidRange
> attempt, a 3pt attempt, or a FTA. I never finished the study, due
to health
> problems, but I still have the data. I'll post it in a day or two -
- if
> Kevin wants to send me his data, I can work that in too.
>
> Ah hell, I'll post what I have right now:
>
> AST UNAST Diff p
> Inside% 65% 47% +18% 0.006
> MidRange% 45% 34% +11% 0.046
> All2pt% 55% 43% +12% <0.000
> 3pt% 38% 21% +17% 0.002
> FT% 68% 75% -7% 0.140
>
> --
>
> ed
> "I ain't the world's best writer, ain't the world's best speller
> But when I believe in something I'm the loudest yeller."
> - Woody Guthrie
• ... From: dan_t_rosenbaum [mailto:rosenbaum@uncg.edu] Sent: Friday, September 17, 2004 9:07 PM [...] ... Not that I know of. It would be valuable information
Message 6 of 12 , Sep 18, 2004
-----Original Message-----
From: dan_t_rosenbaum [mailto:rosenbaum@...]
Sent: Friday, September 17, 2004 9:07 PM

[...]

>Has anyone ever calculated the true shooting percentage for
>potentially assisted field goal attempts? How does it compare to
>the true shooting percentage for all field goal attempts?

Not that I know of. It would be valuable information to have,
because it would help us calculate the typical value of an
assist, assisted FGM, and unassisted FGM.

--MKT
• ... assists, and ... MidRange ... Ed doesn t include the relative frequency of these situations, but the impact is apparent. Potentially-assisted 3-point
Message 7 of 12 , Sep 19, 2004
--- In APBR_analysis@yahoogroups.com, igor eduardo küpfer
<edkupfer@r...> wrote:
>.. I
> charted about 35 games, keeping track of assists and potential
assists, and
> whether the attempt that followed was an "inside" attempt, a
MidRange
> attempt, a 3pt attempt, or a FTA. >
> AST UNAST Diff p
> Inside% 65% 47% +18% 0.006
> MidRange% 45% 34% +11% 0.046
> All2pt% 55% 43% +12% <0.000
> 3pt% 38% 21% +17% 0.002
> FT% 68% 75% -7% 0.140
>

Ed doesn't include the relative frequency of these situations, but
the impact is apparent. Potentially-assisted 3-point attempts
are "worth" 80% more than unassisted 3's.

Crazily, an "assisted FT" is only 90% as redeemable as an unassisted
one. I can only guess there are disproportionate "big men" (poorer
FT shooters) getting the ball from a (better FT-shooting) teammate
and being fouled. Makes sense, really.

Then again, this was Toronto and their opponents, for 35 games.
Right?

I thought this was an intriguing study. It seems to have died
prematurely, right after I interpreted a ridiculously high value for
the assist.

Anyone remember when those posts appeared ?
• ... Here is message #3232, from Feb. 13, 04. The shooting % are identical, so it looks as if Ed only charted 26 games, after all. ... ***************** And
Message 8 of 12 , Sep 19, 2004
--- In APBR_analysis@yahoogroups.com, "Mike G" <msg_53@h...> wrote:
> --- In APBR_analysis@yahoogroups.com, igor eduardo küpfer
> <edkupfer@r...> wrote:
> >.. I
> > charted about 35 games, keeping track of assists and potential
> assists, and
> > whether the attempt that followed was an "inside" attempt, a
> MidRange
> > attempt, a 3pt attempt, or a FTA. >
> > AST UNAST Diff p
> > Inside% 65% 47% +18% 0.006
> > MidRange% 45% 34% +11% 0.046
> > All2pt% 55% 43% +12% <0.000
> > 3pt% 38% 21% +17% 0.002
> > FT% 68% 75% -7% 0.140

Here is message #3232, from Feb. 13, '04. The shooting % are
identical, so it looks as if Ed only charted 26 games, after all.

**************

--- In APBR_analysis@yahoogroups.com, igor eduardo küpfer
<edkupfer@r...> wrote:
> After 26 games, here's what it looks like:
>
> 2m 2a 2% 3m - 3a 3% ftm - fta ft%
> AST 311 - 570 55% 97 - 252 38% 97 - 142 68%
> UNAST 441 - 1037 43% 20 - 95 21% 239 - 318 75%
>
> DIFF 12% 17% -7%
> p <.001 <.01 ns

*****************

And here is my reply at the time:

***************

I've reduced this chart to one that represents an "average" game in
the sample:

.category. 3pt. 2 pt. FT

"assisted" 4-10 12-22 4-6
unassisted 1-5_ 17-40 9-12

Counting 2 FTA as one 'play':

On 35 "assisted" plays, 40 pts are scored, or 1.143 pts per play

On 51 unassisted plays, 46 pts are scored, at .902 ppp

The difference is .216 ppp

But: In reality, only 16 assists are counted. So, from 16 actual,
counted assists, 40 pts are produced.

Therefore, each assist produces 2.5 points...

*****************

(Back to Sep. 19 of '04)

Plays involving a "granted" assist produced 2.5 pts, on average.
The difference between such plays and the average unassisted play is
2.5-.902 = 1.598

Is that the value of an actual, granted assist? Or shall we take
the value of .216 and multiply it by the ratio of granted/potential
assists, which is 35/16 (= 2.19)

2.19 * .216 = .473

I think maybe the higher estimate gives 100% credit to the assister,
and none to the shooter.

(There are other considerations: the "typical" 1-5 shooting on
unassisted 3's probably include a halfcourt heave or 2)

And 2 days after this post, Dan R introduced himself to the group;
and the conversation went elsewhere ...
• Below is a re-post of Ed s work. By my calculations the true shooting percentage is 57.1 percent on assisted true field goal attempts and 46.4 percent on
Message 9 of 12 , Sep 20, 2004
Below is a re-post of Ed's work.

By my calculations the true shooting percentage is 57.1 percent
on "assisted" true field goal attempts and 46.4 percent
on "unassisted" true field goal attempts. In my opinion, this is a
huge difference and suggests that assists are quite valuable.

Note that "unassisted" true field goal attempts would include most
fouls at the end of the game and most uncontested break-away layups,
which might bias upwards the true shooting percentage
for "unassisted" true field goal attempts. On the other hand, I
suspect that when teams are holding the ball at the end of the
quarter or at the end of the game, a disproportionate percentage of
those field goal attempts are "unassisted," which likely biases
downward the true field goal percentage for "unassisted" true field
goal attempts.

*******************************************
THE REST IF FROM ED.
*******************************************

After 26 games, here's what it looks like:

2m 2a 2% 3m - 3a 3% ftm - fta ft%
AST 311 - 570 55% 97 - 252 38% 97 - 142 68%
UNAST 441 - 1037 43% 20 - 95 21% 239 - 318 75%

DIFF 12% 17% -7%
p <.001 <.01 ns

Over the last dozen games I've started tracking shot attempts in the
paint separate from those outside the paint.

In2m - In2a In2% Out2m - Out2a Out2%
AST 89 - 137 65% 69 - 153 45%
UNAST 129 - 275 47% 66 - 192 34%

DIFF 18% 11%
p <.05 ns

We should be able to combine this with the data from the 82games site
which shows shot attempt distance for each player in order to begin
to credit assists properly -- more credit for inside attempts should
go to the assister. By the end of this season, I'll have enough games
scored from a wide variety of teams to know how much of the credit
should go to the passer.

ed

> I don't know about better, but it was, I think, what Dan was
looking for. I
> charted about 35 games, keeping track of assists and potential
assists, and
> whether the attempt that followed was an "inside" attempt, a
MidRange
> attempt, a 3pt attempt, or a FTA. I never finished the study, due
to health
> problems, but I still have the data. I'll post it in a day or two --
if
> Kevin wants to send me his data, I can work that in too.
>
> Ah hell, I'll post what I have right now:
>
> AST UNAST Diff p
> Inside% 65% 47% +18% 0.006
> MidRange% 45% 34% +11% 0.046
> All2pt% 55% 43% +12% <0.000
> 3pt% 38% 21% +17% 0.002
> FT% 68% 75% -7% 0.140
>
> --
>
> ed
> "I ain't the world's best writer, ain't the world's best speller
> But when I believe in something I'm the loudest yeller."
> - Woody Guthrie
• ... I have reservations about using my numbers to come to any firm conclusions. First, the sample dataset is definitely non-random, consisting mostly of
Message 10 of 12 , Sep 20, 2004
dan_t_rosenbaum wrote:

>
> By my calculations the true shooting percentage is 57.1 percent
> on "assisted" true field goal attempts and 46.4 percent
> on "unassisted" true field goal attempts. In my opinion, this is a
> huge difference and suggests that assists are quite valuable.
>

I have reservations about using my numbers to come to any firm conclusions.
First, the sample dataset is definitely non-random, consisting mostly of
Raptors games. More importantly, the official scorers gave out about 15%
more assists than I did. I re-scored a few of these games, using the loosest
possible definition of assist that I could manage, and my assist totals
still fell short of the official.

In another study I had previously found some evidence of a home court bias
in assist scoring. If my impressions drawn from these studies above are
correct, we have two reasons to be extremely wary of using assists in
individual player ratings.

I urge caution in drawing conclusions from all this, but I also urge an
extremely conservative approach to incorporating assists into your ratings,
given that we have good reasons to think these number are inflated.

ed
• Granted, assists are somewhat subjective and the data that you collected are non-random, so there are good reasons for not coming to firm conclusions based
Message 11 of 12 , Sep 22, 2004
Granted, assists are somewhat subjective and the data that you
collected are non-random, so there are good reasons for not coming to
firm conclusions based upon your work. But right now, I think the
default is to assume that the true shooting percentage is the same
for assisted and unassisted field goal attempts. And what evidence
is there for that default assumption? As far as I can tell, there is
no evidence. Thus, if I have to choose between your limited/non-
random study and no evidence, I would have to go with your study.
Would I like something more complete? Of course, but beggars cannot
always be choosers.

And of the two issues, I think the non-randomness is a bigger deal.
Suppose there is no difference in true field goal percentage between
phantom assisted field goal attempts (those you were not able to
chart) and non-assisted field goal attempts. That would only knock a
point or two off the difference in true field goal percentage that
you measured.

So perhaps the Raptors are more efficient on assisted field goal
attempts and less efficient at defending assisted field goal attempts
and so this is just a Raptors effect. Yes, I am uncomfortable
assuming that this is not true, but I am more uncomfortable with the
default assumption for which there is no evidence. For one thing
your results are NOT simply due to sampling variation, because most
of your results are highly statistically signficant. So the argument
has to be that the Raptors on offense and defense are systematically
different from other teams. I am more comfortable with the
assumption that they are about the same than the assumption that the
true field goal percentage is the same for assisted and unassisted
field goal attempts.

Perhaps as a practicing empirical economist, I simply am more used to
making tenuous assumptions and working with messy data, because the
compromises that we have to make working with NBA data are generally
pretty minor to the assumptions economists often have to make in
working with other types of data.

Best wishes,
Dan

--- In APBR_analysis@yahoogroups.com, igor eduardo küpfer
<edkupfer@r...> wrote:
> dan_t_rosenbaum wrote:
>
> >
> > By my calculations the true shooting percentage is 57.1 percent
> > on "assisted" true field goal attempts and 46.4 percent
> > on "unassisted" true field goal attempts. In my opinion, this is
a
> > huge difference and suggests that assists are quite valuable.
> >
>
> I have reservations about using my numbers to come to any firm
conclusions.
> First, the sample dataset is definitely non-random, consisting
mostly of
> Raptors games. More importantly, the official scorers gave out
> more assists than I did. I re-scored a few of these games, using
the loosest
> possible definition of assist that I could manage, and my assist
totals
> still fell short of the official.
>
> In another study I had previously found some evidence of a home
court bias
> in assist scoring. If my impressions drawn from these studies above
are
> correct, we have two reasons to be extremely wary of using assists
in
> individual player ratings.
>
> I urge caution in drawing conclusions from all this, but I also
urge an
> extremely conservative approach to incorporating assists into your
ratings,
> given that we have good reasons to think these number are inflated.
>
> ed
• One thing that drives me crazy about assists is their subjectivity. For instance, as we ve pointed out here, assists are harder to get in Europe than in the
Message 12 of 12 , Sep 23, 2004
One thing that drives me crazy about assists is their subjectivity.
For instance, as we've pointed out here, assists are harder to get in
Europe than in the NBA. Their official scorers are tougher. That
should mean that their assists are more valuable, leading more

Sooo, if Ed is acting more as a Euro-like scorer, does that mean he is
exaggerating the value of assists? Potentially. More of the passes
that are being called assists are in the fuzzy area, rather than
definitive.

I do think that assists have value, adding to a team's shooting
percentage (most teams, though exceptions exist, I'm quite sure). But
I'm guessing that Ed's study exaggerates that effect at least a little
bit.

I have considered ways of accounting for this subjectivity but never
put it into my production methods (it's all in R&D mode).

DeanO

Dean Oliver
When basketball teams start playing Moneyball, this is the book
they'll use!

--- In APBR_analysis@yahoogroups.com, "dan_t_rosenbaum"
<rosenbaum@u...> wrote:
> Granted, assists are somewhat subjective and the data that you
> collected are non-random, so there are good reasons for not coming to
> firm conclusions based upon your work. But right now, I think the
> default is to assume that the true shooting percentage is the same
> for assisted and unassisted field goal attempts. And what evidence
> is there for that default assumption? As far as I can tell, there is
> no evidence. Thus, if I have to choose between your limited/non-
> random study and no evidence, I would have to go with your study.
> Would I like something more complete? Of course, but beggars cannot
> always be choosers.
>
> And of the two issues, I think the non-randomness is a bigger deal.
> Suppose there is no difference in true field goal percentage between
> phantom assisted field goal attempts (those you were not able to
> chart) and non-assisted field goal attempts. That would only knock a
> point or two off the difference in true field goal percentage that
> you measured.
>
> So perhaps the Raptors are more efficient on assisted field goal
> attempts and less efficient at defending assisted field goal attempts
> and so this is just a Raptors effect. Yes, I am uncomfortable
> assuming that this is not true, but I am more uncomfortable with the
> default assumption for which there is no evidence. For one thing
> your results are NOT simply due to sampling variation, because most
> of your results are highly statistically signficant. So the argument
> has to be that the Raptors on offense and defense are systematically
> different from other teams. I am more comfortable with the
> assumption that they are about the same than the assumption that the
> true field goal percentage is the same for assisted and unassisted
> field goal attempts.
>
> Perhaps as a practicing empirical economist, I simply am more used to
> making tenuous assumptions and working with messy data, because the
> compromises that we have to make working with NBA data are generally
> pretty minor to the assumptions economists often have to make in
> working with other types of data.
>
> Best wishes,
> Dan
>
> --- In APBR_analysis@yahoogroups.com, igor eduardo küpfer
> <edkupfer@r...> wrote:
> > dan_t_rosenbaum wrote:
> >
> > >
> > > By my calculations the true shooting percentage is 57.1 percent
> > > on "assisted" true field goal attempts and 46.4 percent
> > > on "unassisted" true field goal attempts. In my opinion, this is
> a
> > > huge difference and suggests that assists are quite valuable.
> > >
> >
> > I have reservations about using my numbers to come to any firm
> conclusions.
> > First, the sample dataset is definitely non-random, consisting
> mostly of
> > Raptors games. More importantly, the official scorers gave out
> > more assists than I did. I re-scored a few of these games, using
> the loosest
> > possible definition of assist that I could manage, and my assist
> totals
> > still fell short of the official.
> >
> > In another study I had previously found some evidence of a home
> court bias
> > in assist scoring. If my impressions drawn from these studies above
> are
> > correct, we have two reasons to be extremely wary of using assists
> in
> > individual player ratings.
> >
> > I urge caution in drawing conclusions from all this, but I also
> urge an
> > extremely conservative approach to incorporating assists into your
> ratings,
> > given that we have good reasons to think these number are inflated.
> >
> > ed
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