Newsgroups: sci.med.dentistry

Subject: Re: Periostat and statistics

Date: Fri, 20 Apr 2001 15:09:15 GMT

Organization: EarthLink Inc. -- http://www.***.com/

Quote:

> Including the standard deviation, or the measure of variance or deviation

> from the mean would clearly show whether or not there is statistical

> improvement. Alas, Periostat fails to do that!

Joel, how do they deteremine the P value? ie. (P>0.05) I was under the

impression that P values were of more value because the took into

consideration standard deviation. This is what all the clinical trial

studies seem to use. Is there something you know that the reviewers don't?

******

Information on standard deviation posted below: Way below!

*****

Quote:

> Can't be. When the mean for N=99 is 0.25 millimeters, then you need a huge

> outlier to alter the average! In fact, you also need negative numbers to

> balance it!

what if N=9999?

Same. If a pocket is measuring 6 mm, and the improvement is 0.25 mm, One

needs negative numbers to balance out a putative 3 mm improvement!

Quote:

> To use the golf analogy, supposing that the par for 18 holes is 65. Now

> plenty of golfers are taking 90 or 100 to play the same 18 holes. This

means

> that some are doing the 18 holes in 20 strokes.

Really, is that the only way you could come up with that number? Had no

idea.

I am a miniature golf player ... I am just faking this part .....

Quote:

> An individual practitioner can never judge efficacy ....... How does he do

> the double-blind thing all by himself?

I assume the same way you would judge the efficacy of of a new cement or

impression material.

Right. I have a vague impression that cement A works well in my hands. As

for patient response to cement A, how would I eliminate confounding

variables?

If you can't trust the research, and you can't trust you own observations,

what can you do?

Read Reality, CRA, etc.

*****

Same with golf scores, same with Periostat data.

Std. Deviation - an unknown to your golfing buddy!

How about printing this out and handing it to him?

***

Hypothetical case:

New York City and Duluth, MN both have average

household incomes of $78,343. Which city would be the

better location for your brand new BriteSmile Dental

Whitening Center? PS- It costs the patients from $500-

$700 for an hour's whitening treatment. Better get a

good grip on household income distribution first!

The Std. Dev. (Standard Deviation) could supply the

missing information needed to make an informed choice.

The "average" for both cities could be identical

however the data could be -- would be --- vastly

different!

Same with golf scores, same with Periostat data.

Joel M. Eichen, D.D.S.

[background post - posted 7/24/99]

Hello!

I received an e-mail from Bob Kehoe asking for some

more information about standard deviation with regards

the article about staff salaries. His magazine has

supplied the mean and the range, but had omitted the

standard deviation. It is a statistic that is readily

available if your data is in an Excel spreadsheet or

any database program. If your data is on 3 by 5 cards,

then you must calculate standard deviation separately.

This is not just a concept for mathematicians and for

scientists. It works for everybody.

It comes down to this:

Some statistics, such as the mean and the range, are

easily calculated, but hide the true pattern of

the numbers that produce them.

The mean describes a set of numbers with a single

central number. In contrast, the range reveals how the

numbers in a set vary from each other. But often these

two statistics do not fairly represent the numbers in

their sets.

It needs a second statistic, the standard deviation, to

make sense of these contradictions.

** ** **

Standard deviation (meaning plain old deviation from

the mean) tells us how much variation there is in the

numbers. How far away from the mean does the "regular"

(whatever regular is) number lie?

Lets say that the data shows that the average hygienist

gets $25. and hour with a std. deviation of $4. an

hour. Two standard deviation units indicates (2 times

$4 or $8) meaning that 95% of all of the doctors

reporting will pay their hygienists between $17. an

hour and $33. an hour (mean plus or minus 2 std. dev).

99% of all of the doctors reporting will pay their

hygienists between $13. an hour and $37. an hour (mean

plus or minus 3 std. dev). The other 1% are known as

outliers. They affect the mean but do not affect the

standard deviation.

The mean (arithmetic average) is a very poor statistic.

Does this statement surprise you?

Here's the temperatures for 3 U.S. cities.

Normal Monthly Mean Temperatures

(Fahrenheit) for 1961-1990

January July Yearly Mean

Fairbanks, AK -1.6 72.3 36.5

Phoenix, AZ 65.9 105.9 85.9

Honolulu, HI 80.1 87.5 84.4

The annual mean temperature for Fairbanks, Alaska is

36.5, but it is a balmy 72.3 degrees in July.

The mean for Phoenix, Arizona is 85.9, yet it is a cool

65.9 in January.

The mean is a poor predictor in these 2 cities, but in

Honolulu, Hawaii the mean of 84.4 is typical of

temperatures year round.

Clearly the mean is not enough.

Suppose that you are in some course and have just

received your grade on an exam. It is natural to ask

how the rest of the class did on the exam so that you

can put your grade in some context. Knowing the mean or

median tells you the "center" or "middle" of the

grades, but it would also be helpful to know some

measure of the spread or variation in the grades.

Lets look at a small example. Suppose three classes of

5 students each write the same exam and the grades are:

Class 1

Class 2

Class 3

82

82

67

78

82

66

70

82

66

58

42

66

42

42

65

Each of these classes has a mean, of 66 and yet there

is great difference in the variation of the grades in

each class. One measure of the variation is the range,

which is the difference between the highest and lowest

grades. In this example the range for the first two

classes is 82 - 42 = 40 while the range for the third

class is 67 - 65 = 2. The range is not a very good

measure of variation here as classes 1 and 2 have the

same range yet their variation seems to be quite

different. One way to see this variation is to notice

that in class 3 all the grades are very close to the

mean, in class 1 some of the grades are close to the

mean and some are far away and in class 2 all of the

grades are a long way from the mean. It is this concept

that leads to the definition of the standard deviation.

Lets look at class 1. For each student calculate the

difference between the students grade and the mean.

Class 1 scores

Difference from the mean

82

16

78

12

70

4

58

-8

42

-24

The average of these differences could now be

calculated as a measure of the variation, but this is

zero. What is really needed is the distance from each

grade to the mean not the difference. You could take

the absolute value of each difference and then

calculate the mean. This is called the mean deviation,

i.e. mean deviation = , where n is the number of

students in the class. For class 1 this is 64/5 =

12.8. Another way to deal with the negative differences

is to square each difference before adding.

Class 1 scores

Difference from the mean

Difference from the mean squared

82

16

256

78

12

144

70

4

16

58

-8

64

42

-24

576

The sum of this column is 1056. To find what is called

the standard deviation, s, divide this sum by n-1 and

then, since the sum is in square units, take the square

root. For class 1 this gives std. deviation.

To see some practical examples, look at the following:

http://www.***.com/

(Medical school admissions, University of British

Columbia, showing mean scores, range and standard

deviation).

http://www.***.com/

le.html

(This one is a standard deviation calculator on the

internet. You can also use a scientific calculator, but

if you do not have one, then go here.)

http://www.***.com/ ~dmason/stat/stat2.html

(A second internet standard deviation calculator)

Cheers,

Joel

Joel M. Eichen, D.D.S.

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

[This post from 2/16/99]

A community

Some folks wonder why I post about so many different topics.

I frequently include such diversity as local politics or the

state of the economy. It is because I posit that a strong

dental profession relies on an entire cadre of patients,

friends, and neighbors for its continuing existence. We hope

that you are enjoying the fruits of a strong financial

position. We need you to be fairly secure in your own

finances to help us support our dental practices. We, in

turn will do our best to care for the state of your teeth.

Corporate consolidators who buy practices and owe fealty to

shareholders are currently presenting a real challenge for

privately owned dental practices. Managed care dental plans

contain challenging aspects too. Perhaps the biggest threat

of all to today's dentistry is the pandemic practice of land

speculation.

The 1998 U.S. average family income is around $34,000.

Unfortunately, this does not begin to tell the story of what

the "average family" has available as income. To determine

that, one needs to know the shape of bell curve and how

steeply family income falls away from the mean (or average).

The concept of "standard deviation" or "variance from the

mean" tells us how many people fall
...

**read more »**