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Statistical analyses in the real world. Paul Williams Lawrence Berkeley National Laboratory. Statistical significance. Theory: The probably of observing something by chance Practice: The accepted threshold P<0.05 is publishable P>0.05 is not publishable

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statistical analyses in the real world

Statistical analyses in the real world

Paul Williams

Lawrence Berkeley National Laboratory

statistical significance
Statistical significance
  • Theory: The probably of observing something by chance
  • Practice: The accepted threshold
      • P<0.05 is publishable
      • P>0.05 is not publishable
      • The promise to produce P<0.05 is a primary consideration for NIH funding
choice of an appropriate statistical test
Choice of an appropriate statistical test
  • Familiarity to the reader, reviewer, granting agency
  • Succinctness, in consideration of journal space, simplicity in presenting results
  • Statistical tests promise the world, which we can then promised to funding agencies, journals, etc.
relationship of average bmi to weekly distance walked in women
Relationship of average BMI to weekly distance walked in women

kg/m2

Williams PT.Med Sci Sports Exerc. 2005 371893-901.

relationship of percentiles of the bmi distribution to weekly distance walked in women
Relationship of percentiles of the BMI distribution to weekly distance walked in women

kg/m2

Williams PT.Med Sci Sports Exerc. 2005 371893-901.

regression slope by percentile
Regression slope by percentile

Slope (kg/m2 per km/wk)

Williams PT.Med Sci Sports Exerc. 2005 371893-901.

decline in bmi per km wk run or walked
Decline in BMI per km/wk run or walked

Slope (∆BMI per 1km difference in

weekly distance)

Williams PT.Med Sci Sports Exerc. 2005 371893-901.

q q plot of female runners and walkers bmi distribution
Q-Q plot of female runners’ and walkers’ BMI distribution

Walkers’ BMI distribution

estimated change in bmi per one km wk difference in running distance
Estimated change in BMI per one km/wk difference in running distance

Slope (∆BMI per 1km difference in

weekly distance)

published applications
Published applications:

“… the effects of physical activity, alcohol, and weight

reduction on HDL-C levels may be, to a large extent,

dependent on the initial level with the greatest

improvement achieved in subjects with high HDL

and the least improvement in those having low

HDL-C levels.”

Williams PT.The relationships of vigorous exercise,

alcohol, and adiposity to low and high high-density

lipoprotein-cholesterol levels.Metabolism.

2004 Jun;53(6):700-9.

published applications1
Published applications:

“We speculate that the reported greater increases in

triglycerides per unit of adiposity in whites than blacks,

in men than women, and in low-density lipoprotein

(LDL) pattern B than A are all consistent with the

relationships we observe.”

Williams PT. Relationship of adiposity to the population

distribution of plasma triglyceride concentrations in

vigorously active men and women. Atherosclerosis.

2004 Jun;174(2):363-71.

published applications2
Published applications:

“We speculate that the reported greater increases in

triglycerides per unit of adiposity in whites than blacks,

in men than women, and in low-density lipoprotein

(LDL) pattern B than A are all consistent with the

relationships we observe.”

Williams PT. Relationship of adiposity to the population

distribution of plasma triglyceride concentrations in

vigorously active men and women. Atherosclerosis.

2004 Jun;174(2):363-71.

published applications3
Published applications:

“These results are consistent with the hypothesis that

running promotes the greatest weight loss specifically

in those individuals who have the most to gain from

losing weight.”

Williams PT. Vigorous exercise and the population

distribution of body weight.Int J Obes Relat Metab

Disord. 2004 Jan;28 (1):120-8

slide20
After assigning significance, the second most important contribution of statistics to research scientist is adjustment
  • For example, walkers may be leaner than nonwalkers but is it because they eat better.
  • Statistical adjustment is usually a sufficient argument for journals, funding agencies, etc.
classical statistical adjustment
Classical statistical adjustment

Triglycerides (mmol/L)

alternative statistical adjustment
Alternative statistical adjustment

% reduction

Williams PT.Metabolism

. 2004;53:700-9.

3 analysis of change data

3. Analysis of change data

DBMI=a +bDdistance

d bmi a bd distance
DBMI=a +bDdistance

Cross-sectional

relationship

BMI

Distance

translating change data into a relationship
Translating change data into a relationship

Doesn’t correspond

To:

DBMI=a +

bDdistance

gDdistance2

Cross-sectional

relationship

BMI

The amount of change depends upon the starting and ending distance

exposure model relating d bmi to change in running distance
Exposure model relating DBMI to change in running distance

Estimated DBMI due to a “dj-cj” km/wk change

in running distance

Williams PT, Wood PD. Int J Obes (Lond). 2006 30:543-51

original interpretation of acls
Original interpretation of ACLS

Blair SN, et al. JAMA. 1995;273:1093-8.

variables measured with error
Variables measured with error

Second fitness measurement (Treadmill duration)

Williams PT. Med Sci Sports Exerc. 2003;35:736-40.

our interpretation of acls
Our interpretation of ACLS

Fitness measured

Fitness measured

Williams PT. Med Sci Sports Exerc. 2003;35:736-40.

simulation versus reported results
Simulation versus reported results

Williams PT. Med Sci Sports Exerc. 2003;35:736-40.

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