EPIB 601: Quiz 1

Lectures 1-3

What distinguishes a ratio?

• A/B

- Dimensionless or dimensions

What distinguishes a proportion?

• A/(A+B)

• 0, 1

• Dimensionless

What distinguishes a rate?

• N/T (instantaneous change over time)

• Has dimensions

• E.g., speed

Two categories of incidence measures?

1. Cumulative incidence/risk/incidence proportion/attack rate

2. Incidence density/incidence rate

Synonyms of incidence proportion

• Cumulative incidence

• Risk

• Attack rate

Formula of incidence proportion

new cases during followup/#people followed-up

What are the assumptions of incidence proportion?

• Conditional on no competing risks

• Timing is everything for the interpretation (over a year? 2 years?)

• Fixed cohort with no exits

Interpretation of incidence proportion?

Risk/probability of developing a disease

Formula of incidence density

new cases during follow-up/total person-time at risk during follow-up

Unit of incidence density

Time^(-1)

Incidence density is a…

Rate (like speed)

What is incidence density useful for?

• Dynamic cohorts

- Dealing with competing risks

Problem with incidence density?

Person-years are considered equivalent, but they are not, making it hard to compare studies

Interpretation of incidence density?

If in a steady population:

• Put over 1 year

• Take reciprocal (1/xx)

• Interpret as average time until event occurs

Risk Incidence rate?

Risk = (Incidence Rate)*Time

• Good if risk is < 20%

• Becomes less good as we go through years, it becomes an overestimate

When are survival analyses good?

• Dynamic cohort

- Changing incidence rates

Assumptions of survival analyses?

• No changes in survivorship over calendar time

- Uniform withdrawal/events over interval

Formula for prevalence proportion?

existing cases/#total population

Incidence Prevalence

If steady state:
- P/(1-P) = (Incidence rate)Duration of disease
If rare disease (P < 0.10)
- P = (Incidence rate)Duration of disease

Direct standardization

Apply rates from population of interest to the standard population

Standardized mortality ratio formula

= observed events/expected events x 100

7 steps of natural history of disease?

1. Biological onset

2. Pathological evidence

3. Symptoms

4. Medical care sought

5. Diagnostic

6. Treatment

7. Outcome (cure, control, disability, death)

According to the natural history of disease, what is the preclinical phase? and the clinical phase?

Preclinical: from biological onset to symptoms
Clinical: from symptoms to outcome

According to the natural history of disease, when does primary prevention takes place and what is it?

• Before biological onset

- Attempt to prevent development (e.g., vaccination)

According to the natural history of disease, when does secondary prevention takes place and what is it?

• Between biological onset and symptoms appearance

| - Screen to improve prognosis

According to the natural history of disease, when does tertiary prevention takes place and what is it?

• Between appearance of symptoms and outcome

| - Management to diminish impact of disease

What is sensitivity and what does it result in?

• P(T+ | D+)

| - Higher sensitivity means fewer false negatives

Formula for sensitivity?

= True positives / all D+

What is specificity and what does it result in?

• P(T- | D-)

| - Higher specificity means fewer false positives

Formula for specificity?

= True negatives / all D-

What happens when you require T+ on two tests to be considered D+, such as in sequential testing?

Less Sensitivity = Sensitivity1 x Sensitivity2 (you end up with more false negatives)
More specificity = Specificity1 + Specificity2 - (Spec1 x Spec2) (you end up with less false positives)

What happens when you require T+ on one of two tests to be considered D+, such as in simultaneous testing?

More sensitivity = Sensitivity1 + Sensitivity2 - (Sens1 x Sens2) (you end up with less false negatives)
Less specificity = Specificity1 x Specificity2 (you end up with more false positives)

Positive predictive value formula

P (D+ | T+) = True positives/all T+

Negative predictive value formula

P (D- | T-) = True negatives/all T-

Relation between predictive value and prevalence?

Increased prevalence leads to increased PPV

Relation between Sensitivity/specificity and predictive value?

Specificity impacts PPV

| Sensitivity impacts NPV

Formula for PPV with sensitivity and specificity?

PPV = (sens x prev)/[(sens x prev) + (1-spec)*(1-prev)]

Formula for NPV with sensitivity and specificity?

NPV = (spec x 1-prev)/[(spec1-prev)+(1-sens)prev)]

What is a likelihood ratio?

Likelihood that a test result would be expected in a person D+
Compared to
The likelihood that the same result would be expected in a person D-

How is LR+ calculated?

P(true positives)/P(false positives) = sens/(1-spec)

How is LR+ interpreted?

The higher the LR+, the more likely the test is positive in D+ person compared to D- person (rules in the disease)

How is LR- calculated?

P(false negatives)/P(true negatives) = (1-sens)/spec

How is LR- interpreted?

The lower the LR-, the less likely the test is negative in a D+ person compared to a D- person (rules out the disease)

What are significant numbers for LR?

LR = 1, NS
LR+ > 1 --> associated with disease (good from > 10)
LR- < 1 --> associated with no disease (good from < 0.1)