From: halpin@vax.oxford.ac.uk
Newsgroups: alt.activism
Subject: Re: Harel gets a defender (was Re: Yet another pathetic Friedman post
Date: 3 Oct 92 12:36:13 GMT
Organization: Oxford University VAX 6620

Usenet Tennis between Michael Friedman (mfriedma@uucp) 
and me (halpin@vax.oxford.ac.uk):

Key:
  Harel: >>>>
  Michael: >>> and >
  Brendan: >> and new text

[First, Harel quotes poverty figures which are simply wrong -- a
typo or two, I presume -- and then goes on to interpret them. But
the present argument is about the maths and not the figures, so we
continue...]

>>>>        [That would make 15.5% of people in poverty Black -- Blacks 
>>>>        are 12% of the population; hence poverty for Blacks is about 
>>>>        30% higher than the (already abysmal) national average. 
>>>>        --Harel] 

...

>>> If these numbers were true the poverty level for blacks would be .033%
>>> compared to a national average of .025%.  The poverty rate for blacks
>>> would be .008% higher than the national average.
> 
>>> You are taking a percent of a percent, something that is
>>> not meaningful in this context.

[Arguments that Harel is right and that 0.008% is wrong and
misleading deleted]

> Also, I think the .008% is a much more useful figure.
> 
> After all, this isn't like the death penalty where we are focusing in
> on the particular people involve.  We're using this stat as a way of
> getting a view of the entire population.  In that case, the .008%
> number tells us that the poverty level shows no real difference
> between the two groups, and that's right - after all, it includes so
> few people that it isn't significant.
> 

This is *absolutely* wrong. 

The only reason the figure is ridiculously small because it is
based on Harel's incorrect figures (wrong, I would guess, by a
factor of between 100 and 1000). A correct figure would be of the
order of 5%, and would represent as a %age of the whole black
population, that part of it which is in poverty but would not be if
the black population had the average poverty rate. Let's say 5% is
correct and the population of the US is 250m, 12% black. This comes
out to 1,500,000 people.

That is, if you could magically reduce the black poverty rate to
the population average, 1.5 million people would no longer be poor.
Very insignificant, to use your term.


Now, we can to the arithmetic that way (hello, John :-) but it
doesn't inform us about the relative position of black people. The
summary figure I want to see (after the absolute level of poverty)
is something that tells me how much better or worse a particular
group's chance of being poor is. We can do this equivalently by
dividing its poverty rate by the general poverty rate (your figs:
0.033%/0.025% = 1.32), or by dividing its share of the poor
population by its share of the whole population (Harel's figs:
15.5%/12% = 1.29, difference is rounding error). 

But I think you already agree with this:

> Correct, but that's not what Harel said.

So, why is it when Harel inserts a terse statement:

>>>>        [That would make 15.5% of people in poverty Black -- Blacks 
>>>>        are 12% of the population; hence poverty for Blacks is about 
>>>>        30% higher than the (already abysmal) national average. 
>>>>        --Harel] 

that I read it to be making this simple clear point (blacks are
over-represented in poverty by a factor of 1.3 -- you understand
the relationship between ``30% higher'' and ``a factor of 1.3''?),
and you, alternatively, read it to mean something bizarre like that
an extra 30% of the whole black population is in poverty.

Okay, it's easy to get confused when people start to use lots of
percentages, but, frankly, it seems to me that in your attempts to
trip Harel up, you're falling over your own feet.


Brendan

Brendan Halpin                    Email: HALPIN@VAX.OXFORD.AC.UK
Nuffield College                  Phone: +44 865 278642
Oxford OX1 1NF
United Kingdom