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به نام ايزد يكتا. دكتر داودخليلي. Cut points of OBESITY. Dr. Khalili PhD candidate in epidemiology Shahid beheshti university (MC). Some Points About Dichotomizing continuous predictors. Trade off. Simplicity & Practicality . Measurement error & low Power. We loss some information

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به نام ايزد يكتا

دكتر داودخليلي

cut points of obesity
Cut points ofOBESITY

Dr. Khalili

PhD candidate in epidemiology

Shahidbeheshti university (MC)


Some Points About Dichotomizing continuous predictors

Trade off

Simplicity & Practicality

Measurement error & low Power

  • We loss some information
  • Throwing about 1/3 of data away
  • Avoid of assumptions
  • Calculate better effect measurements

Some Points About Dichotomizing continuous predictors

Dichotomizing: to create two relatively homogenous group

According to variable distribution


85% or …of percentile

Using a gold standard

(usually another variable or event )


Cut points based on a Gold Standard

  • Receiver Operating Characteristic Curve (ROC)

AUC (area Under the Curve)


Cut points based on a Gold Standard


Sensitivity 1

1 - specificity


Sensitivity + Specificity -1



Cut points based on a Gold Standard

  • Calculating an effect measure (OR, RR, HR, …)
  • To compare with a reference
  • Agreement Chart

National Health and Nutrition

Examination Surveys




NHES I 1960-62 18-79 years

NHES II 1963-65 6-11 years

NHES III 1966-70 12-17 years

NHANES I 1971-75 1-74 years

NHANES II 1976-80 6 mo.-74 years

HHANES 1982-84 6 mo.-74 years

NHANES III 1988-94 2 mo. +

NHANES 1999- All ages


nih consensus conference 1985
NIH consensus conference (1985):
  • According to NHANES II and85th percentile values (men and women ages 20-29 y)
  • BMI of 27.8 for men
  • BMI of 27.3 for women

Probloms of this statistical approach:

  • Distribution Changes
  • Theoretical Curves
  • Need of more information on BMI complication
  • Low sensitivity because of underestimation of Obesity

Age-adjusted trends in obesity (BMI >=30): United States

1995 who expert committee report
1995 WHO expert committee report

BMI cut-points of 25 (overweight) and 30 (obesity) recommended by expert committees

  • For adults, the Expert Committee proposed classification of BMI with the cut-off points 25, 30 and 40…This classification is based principally on the association between BMI and mortality.
relation between mortality and bmi
Relation between mortality and BMI

Data from Lew EA: Mortality and weight: insured lives and the American Cancer Society studies. Ann Intern Med103:1024-1029, 1985.


The method used to establish BMI cut-off points has been largely arbitrary. In essence, it has been based on visual inspection of the relationship between BMI and mortality: the cut-off of 30 is based on the point of flexion of the curve.


1998 NHLBI (National Heart, Lung, and Blood Institute )

  • Clinical Guidelines

In this report, overweight is defined as a BMI of 25.0 to 29.9 kg/m2 and obesity as a BMI of 30 kg/m2. The rationale behind these definitions is based on epidemiological data that show increases in mortality with BMIs above 25 kg/m2. The increase in mortality, however, tends to be modest until a BMI of 30 kg/m2 is reached.


Recent study in western Europe and North America

“Body-mass index and cause-specific mortality in 900 000 adults:

collaborative analyses of 57 prospective studies”

BMI is in itself a strong predictor of overall mortality both above and below the apparent optimum of about 22·5–25 kg/m2. The progressive excess mortality above this range is due mainly to vascular disease and is probably largely causal. At 30–35 kg/m2, median survival is reduced by 2–4 years; at 40–45 kg/m2, it is reduced by 8–10 years (which is comparable with the effects of smoking). The definite excess mortality below22· 5 kg/m2 is due mainly to smoking-related diseases, and is not fully explained.

Lancet. 2009 March 28; 373(9669): 1083–1096.


Ischaemic heart disease and stroke mortality versus BMI in the range 15–50 kg/m2

Lancet. 2009 March 28; 373(9669): 1083–1096.

wc cut points
WC cut points
  • According to:
  • - Lean MEJ, Han TS, Morrison CE. Waist circumference as a measure for indicating need for weight management. BMJ 1995;311:158–61.

Randomly recruited 904 men and 1014 women,

aged 25 to 74 years, from the general population of

north Glasgow between January and August 1992,

excluding only those who were chair bound.


BMI as

Gold Standard

Using in ATPIII & EGIR


These cutpoints have been shown, in a random sample of 2183 men and 2698 women from the Netherlands, to be associated crosssectionally with an adverse cardiovascular risk profile.

T S Han, EMvan Leer, J C Seidell, ME J Lean

Waist circumference action levels in the identification of cardiovascular risk factors: prevalence study in a random sample. BMJ, 1995;311:1401-5


BMI 25 at action level I or 30 at action level 2

as Gold standard

T S Han, EMvan Leer, J C Seidell, ME J Lean



ShanKuan Zhu,

Am J ClinNutr2002;76:743–9.

Current WC cutoffs proposed by the National Institutes of

Health and the World Health Organization were not chosen on the

basis of their empirical relation to risk factors. Rather, these cutoffs

were derived by identifying WC values corresponding to BMI

cutoffs for overweight (BMI = 25) or obesity (BMI = 30) (2, 21_).

If WC has an independent or a stronger association with risk factors

than BMI has, then it is inappropriate to base WC thresholds

on their association with BMI thresholds. Rather, thresholds for

each should be based on their relation to risk factors. Hence, existing

cutoff recommendations may not take full advantage of the

relation between WC and obesity-related cardiovascular disease

risk factors.

country ethnic specific values for wc a consensus statement from the idf
Country/ethnic-specific values for WCA Consensus Statement from the IDF

Diabet. Med. 23, 469–480 (2006)

cohort studies to determine wc cutoff
Cohort Studies to determine WC cutoff




Cut off


Brazil HTN ----- M:87 F:80 2009

Australia CVD mortality 20-69 M:96 F:80 2007

Japan CVD ≥ 40 M:90 F:80 2009

Thailand CHD 35-59 M:82 2007

China* CVD risk 18-93 M:83-88F:76 2007

Iran CVD ≥ 40 M:94.5 F:94.5 2009

different gold standard
Different Gold Standard

Different Cut points

The more Hard Outcome with lower prevalence

One prevalent CVD rick factor

Two prevalent CVD rick factor

Three prevalent CVD rick factor

Incident CVD

CVD mortality

The higher Cut point