Answering Biostatistics questions for IFOM and the USMLE: Part 1

Let us look at the sample NBME question below:

A community public health department has a limited budget for new interventions and must decide between two options. Option A is to reduce exposure to an industrial chemical that increases the risk for leukemia from 0.5 per 100,000/year to 2.0 per 100,000/year. It is estimated that 30% of the working population in the community is exposed to this agent. Option B is to reduce exposure to a different toxin that increases the risk for aplastic anemia from 0.5 per 100,000/year to 50 per 100,000/year. It ¡s estimated that 5% of the working population is exposed to this toxin. The estimated cost of each intervention is US $740,000. It is assumed that each intervention program will have similar effectiveness in eradicating the exposure. The case fatality rates are similar for both diseases. Which of the following is the best rationale for the health department to use in selecting an option?

A) Option A because more fatalities will be prevented

B) Option A because more workers are exposed to the toxic agent

C) Option B because more fatalities will be prevented

D) Option B because more workers are exposed to the toxic agent

E) The best approach cannot be determined based on the information provided

- This question targets the topic: Relative Risk [Relative risk is a high yield topic]

- The quick formula for Relative Risk for this purpose is to divide:

 

- What these kinds of questions ask, is for you to compare the risk of Option A relative to Option B.

- Every other information in the vignette isn’t needed to arrive at the answer.

- You can easily spot this type of question by looking for the keyword “Risk” when you are asked to compare one group to another (Just like the above question).

Solving for an answer:

Exposed group = 2

Unexposed group = 0.5

Putting in the formula:

2/0.5 = 4% (Relative Risk of 4)

 

Exposed group = 50

Unexposed group = 0.5

Putting in the formula:

50/0.5 = 100% (Relative risk of 100)

So the Risk in Option B is 100% relative to that in Option A which is 4%. 

It means it would be better to invest in an intervention that would reduce the risk of disease by 100% than to invest in one that reduces the risk by only 4%.

 

It is not “D) Option B because more workers are exposed to the toxic agent” because we are working with the risk increased per year in each option (i.e 100,000/yr), we are not working with the percentages of workers exposed to the agents. The reason is, even though 30% of workers are exposed to Option A’s agent the risk of getting leukemia is 4%. For Option B, only 5% of the working population is exposed to it, but the risk of getting aplastic anemia is 100% from the calculation.

 

Take home:

When you meet any biostat question that wants you to compare risk in one group to another,

- Remember relative risk

- Extract exposed numbers and unexposed numbers from the questions

- Divide E (exposed) with U (unexposed) [E/U = RR]

- The group with the highest number has the more risk and vice versa.

- Every other information from the vignette may not be helpful

- All the methods or assumptions in these questions might not be perfect but if you grasp the concept, you'll find it easy to answer similar questions.

Reference:
1. Topic: Relative Risk. Book: First Aid for the USMLE step 1 2019. Public Health Sciences/Biostatistics section.

Many of these factors are related, and the systemic factors mostly affects wound healing by influencing the local factors.

 

Factors affecting wound healing can be categorized into 

 

1. Local factors (VON PIE)

- Local factors are those that directly influence the characteristics of the wound itself.

 

2. Systemic factors (GASIS DOMAIN)

- Systemic factors are the overall health or disease state of the individual that affect his or her ability to heal

 

Local factors (VON PIE)

1. Oxygenation
    Wound that has proper oxygen supply from the vessels and the     
    surrounding, heal properly in due time.
 

    Infection may slow the healing process, by direct effects, causing
    immune reaction and necrosis of surrounding tissue.
    
3. External/Foreign body

    Presence of foreign bodies (for example a wound not properly debrided)
    may be a source of infection and inflammation.

4. Venous Insufficiency

    In venous insufficiency, there is poor drainage of tissue, deoxygenated
    blood with a pile of metabolic waste pools around the vessels and tissues,
    worsening the condition of the tissues and slowing regeneration. 

5. PVD

    When arteries are pathologically narrowed, there is reduced flow of blood
    to the distal areas beyond the narrowing.

6. Peripheral Neuropathy
    If the patient looses ability to feel, he would not be able to shift pressure.
    Prolonged pressure on an area might worsen an uncomplicated injury.
    Blood vessels may also not function properly when they loose they nerve
    supply.

Mnemonic: VON PIE
  Venous insufficiency 
  Oxygenation
  Neuropathy
  Peripheral vascular diseases
  Infections
  External/Foreign bodies

 

1. Gender and Sex hormones
2. Age

3. Stress

4. Ischemia

5. Smoking

6. Diseases: Diabetes, Keloids, Fibrosis, Hereditary healing disorders and collagen diseases, Jaundice, Uremia

7. Obesity

8. Medications: Glucocorticoids, NSAIDS, Chemotherapy (Bevacizumab etc)

9. Alcoholism

10. Immunocompromised conditions: Cancer, Radiation therapy, AIDS

11. Nutrition

Mnemonic: GASIS DOMAIN