Dr. Alaa Hassan - MB BCh, MSc (PH), MD

What Is Normal?

Dr. Alaa El Dine HassanMB BCh, MSc (PH), MD

Consultant- Community and Preventive Medicine

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

Can You Define this “Thing”?

You can define

If Yes

Normality

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

1. The Common

• Normal is synonymous with “Common" . • The (95% probability limits) constitute

the most widely accepted normal range.The normal height of young men is the range of

heights that includes 95% of men.

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

2. Not Ill

• Clearly a definition of illness would have to be devised.

• Illness might thus be defined as anything that causes – incapacitation – pain – swelling – bleeding, or – infection

But is sleep then abnormal?

Would child-birth constitute ill-health?

Is an erection a surgical emergency?

Would menstruation be a sickness?

Is colonic fermentation abnormal?

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

3. Operating as Intended

• The muscles are designed to contract• The bones are designed to support

• What is the intention of axillary hair? • what the appendix is designed to do?• what is the purpose of our nails?

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

4. Conforming to Cultural Norm

• Acceptable Weight standards may vary according to culture

• Nose piercing is normal for Hindu• Nudity is the norm in some African

tribes

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

5. The Best

• desirable/optimal/favorable/advantageous

• Maximization of the useful life-span so that, the degenerative diseases of old age become the commonest causes of death.

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

6. The Therapeutic Definition

• The McMaster Group provide this sixth definition of normality, which is not used in conversational English, but which has great clinical utility.

• Here normality is that range of values (of e.g. arterial blood pressure, or blood sugar) where therapy is not indicated (does more harm than good).

• Patients are therefore not labeled abnormal unless they are to be treated.

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

What’s Exactly Your Point here?

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

If the definition of normality is so controversial ,

how difficult would it be for a TEST to distinguish normal from abnormal ?

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

Diagnostic Tool

• Any procedure (X-ray, Lab measurement, questionnaire, Scale etc..) which aims to differentiate Normal from abnormal.

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

Evaluating a Diagnostic Tool

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

A diagnostic tool is evaluated in terms of two distinct features. Namely:

ValidityReliability(Precision)

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

1. Reliability of Diagnostic Tool

• The Percent of agreement or consistency between multiple observations, measurements, tests or responses in regard to the same unit of observation.

• Repeatability• Giving the same result when applied

several times to the same person.

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

Types of Reliability

Inter-test reliability

Inter-observer reliability

Intra-observer reliability

Respondent Reliability

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

Difference between results obtained on the same person by two or more observers

Inter-observer reliability

Physician 1 Physician 2Same Patient (Blood Pressure)

140/90 130/80

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

Difference between the results obtained by the same observer on the same person

Intra-observer reliability

6:00 pm 6:00 pmSame Patient

Same Physician(Blood Pressure)

140/90 130/80

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

Difference between readings of two tests on the same case or specimen.

Inter-test reliability

Digital Manometer Mercury ManometerSame Patient

(Blood Pressure)

140/90 130/80

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

A special type of inter-observer reliability where there is a difference between the response of the same individual to the same question posed by two different interviewers

Respondent Reliability

Interviewer 1 Interviewer 2(Same Patient )

Sexual Satisfaction Question

YES NO

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

Measurement of reliability

To measure Reliability of blood Pressure measurement , two observers were to measure the systolic blood pressure of the same 10 patients.

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

Subjects Observer 1 Observer 2 Agreement

A 90 90 +

B 90 94 -

C 85 84 -

D 88 88 +

E 92 92 +

F 80 82 -

G 85 85 +

H 98 95 -

I 98 100 -

J 90 90 +

Agreement = 5/10 X100 = 50%

Kappa

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

2. Validity

• This is the accuracy of the test• The ability of the test to measure what it is

intended to measure.

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

Aspects of Diagnostic tool Validity

Validity

1. Face Validity

2. Content Validity

3. Construct Validity

4. Criterion Based Validity√

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

1. Face Validity• Not validity in the technical sense • It refers, not to what the test actually

measures, but to what it appears superficially to measure.

• Based on intuitive judgement • The test "looks valid" to the examinees

who take it, the administrative personnel who decide on its use, and other technically untrained observers.

Social & Behavioral

sciences

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

2. Content Validity

• Also known as logical validity• Refers to the extent to which a measure

represents all aspects of a given Disease . • For example, a depression scale may lack

content validity if it only assesses the affective aspect of depression but fails to take into account the behavioral aspect.

Social & Behavioral

sciences

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

3. Construct Validity

• Evaluation of construct validity requires examining the correlation of the measure being evaluated with variables that are known to be related to the construct (Concept) measured by the instrument being evaluated or for which there are theoretical grounds for expecting it to be related (Campbell & Fiske, 1959)

Social & Behavioral

sciences

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

Construct Validity: A. Convergent Validity

• The scores of a test which evaluates mathematical skills can be correlated with scores on other tests that measure basic mathematical ability,

• High correlations would be evidence of convergent validity.

Social & Behavioral

sciences

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

Construct Validity:B. Discriminant Validity

• To show the discriminant validity of a test of math skills, we might correlate the scores of our test with scores on tests of verbal ability, where low correlations would be evidence of discriminant validity.

Social & Behavioral

sciences

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

4. Criterion Based Validity

• A defined population is subjected to – The test under investigation and – A standard test (Gold Standard)

• The gold standard will determine – Those who are diseased and – Those who are disease free

• The performance of the new test is compared to the Classification of the gold standard test

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

Criterion Based Validity

SensitivitySpecificity

Positive Predictive value

Negative Predictive value

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

Positive Likelihood Ratios

• likelihood ratios are not yet commonly reported in peer-reviewed literature or in marketing information provided by test manufacturers.

• They can be a valuable tool for comparing the accuracy of several tests to the gold standard, and

• They are NOT dependent upon the prevalence of disease.

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

The positive LR represents :

the odds of a positive test in an infected population

compared to

the odds of a positive test a non infected population.

TP/ All Diseased ————--------—

FP/ All Disease free

Or it can also be expressed as : sensitivity—————

1-specificity

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

DiseasedNot

Diseased

Test Positive

TP+FP(All Test Positive)

Test Negative

TN+FN(All Test Negative)

TP+FN(All Diseased)

TN+FP(All Disease Free)

TP

TN

FP

FN

Positive Likelihood Ratio

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

• Useful tests have larger positive LR • Less useful tests will have smaller positive

LR.

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

Example:

A test with a positive LR of 5.0 :– For every

• 1 non-infected subjects that test positive • 5 of the infected subjects will test positive.

– For every • 1 false positive• 5 true positives

– True positives are 5 times False Positives

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

The Negative LR represents :

the odds of a Negative test in an infected population

compared to

the odds of a Negative test in a non infected population.

FN/ All Diseased------------—————TN/ All Disease free

Or it can also be expressed as : 1- Sensitivity

—----------------——Specificity

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

DiseasedNot

Diseased

Test Positive

TP+FP(All Test Positive)

Test Negative

TN+FN(All Test Negative)

TP+FN(All Diseased)

TN+FP(All Disease Free)

TP

TN

FP

FN

Negative Likelihood Ratio

Dr. Alaa Hassan - MB BCh, MSc (PH), MD

• Useful tests have negative LR close to 0• Less useful tests have higher negative LR.