Biostatistics Case Studies

CASE 8: Cross-sectional Study

Clinical History:

A 21-year-old woman from Bangladesh has had decreasing vision in her left eye "for years." Physical examination reveals left corneal and conjunctival scarring. Her eyelashes are partly turned in, scratching the surface of the eye. A conjunctival scraping is made for diagnosis.

A study is performed of 1,000 persons living in this woman's community. It is found that only 55% of persons have access to running water inside their homes. Of those persons with access to running water, 10 had developed the eye disease that the woman in this case had, while 90 persons without running water in their homes had developed the disease.

Microscopic Pathology:

There is an intracytoplasmic elementary body of Chlamydia trachomatis in the cell to the left of the arrow.


  1. What is the diagnosis?

  2. This is trachoma, a chronic, progressive infection of the eye with Chlamydia trachomatis infection, which produces scarring of the conjunctiva and cornea leading to partial or complete blindness.

  3. What are risk factors?

  4. Trachoma is the most common cause for preventable blindness worldwide. Persons living in overcrowded rural communities with limited access to water and health care are at greatest risk. Trachoma is passed easily from person to person in a cycle of infection and re-infection. Mothers of young children are particularly susceptible, and the majority of persons affected are women.

  5. What is the treatment?

  6. Trachoma typically begins in childhood and progresses for years with repeated infections. Infection causes irritation and scarring on the conjunctiva of the inner eyelid, known as trichiasis. Trichiasis causes the eyelashes to turn inward and repeatedly scratch the surface of the cornea, resulting in permanent scarring and eventual permanent blindness. Blindness usually occurs in middle age. The inturned eyelashes can be plucked out, but they will regrow. A surgical procedure can reverse the inturned eyelashes. However, prevention consists of frequent face washing. The early stages of infection can be treated with topical tetracycline or azithromycin.

  7. Name another cause for preventable blindness.

  8. Vitamin A deficiency, which primarily affects children.

  9. What type of study was performed in this case? What is the relative risk for developing this disease for persons without access to running water at home?

  10. This is a cross-sectional study, in which a single group of subjects is recruited and then classified by two criteria (row and column). The criteria in this case are access to running water at home and development of trachoma.

    The table becomes as follows:

    GroupTrachoma presentTrachoma absent
    No access to water90360
    Access to water10540

    The relative risk can be calculated as:

       RR = (a / a + b) / (c / c + d)
          = (90 / 90 + 360) / (10 / 10 + 540)
          = 11

    A Chi-square can be calculated by taking the computation of twice the difference between observed and expected values divided by expected value for each cell, adding them up for all the cells, and comparing to a table of values based upon "degrees of freedom" which is (rows -1)(columns - 1)

    In this case the overall rate of trachoma is 10%, so it would be expected that 55 persons with access to water, and 45 persons without access to water, would have trachoma. Thus, the table's cells of observed (expected) are as follows:

    GroupTrachoma presentTrachoma absent
    No access to water90 (45)360 (405)
    Access to water10 (55)540 (495)

    Which computes to:

    GroupTrachoma presentTrachoma absent
    No access to water2.000.22
    Access to water1.630.18

    The total for all cells is 4, and this is more than the value of 3.841 given in a table of chi square values for 1 degree of freedom for a probability of < 0.05, so this difference between the groups is significant.