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WHAT IS RECALL
BIAS?
The first kind of bias is called recall bias or reporting bias. It pertains to the bias that a patient allegedly has when answering the following question, “Did you ever have an induced abortion?” Some scientists have theorized
that breast cancer patients are more likely to honestly report any abortions that they might
have had than healthy women are (i.e. a bias between breast cancer patients and healthy women).
This theory has been offered as an explanation for the studies linking abortion with breast
cancer. If this phenomenon exists, then the findings in those studies reporting an increased
risk of breast cancer among women having an abortion history -- as opposed to a decreased risk
among women without an abortion history -- would not be authentic.
If this hypothesis is true, then any epidemiological study relying on retrospective data, as opposed to prospective data could be faulty.
Retrospective data are obtained through interviews and/or questionnaires by looking back at
women’s reproductive history. On the other hand, prospective data are obtained through the
use of medical records recorded when an abortion takes place.
It has been argued that if there was such a phenomenon as recall bias, then many thousands of epidemiological studies examining the relationship between a
disease and any controversial risk factor could be called into question. [1, p. 63] For
example, studies examining any one of the following risk factors would be problematical:
1) the link between cervical cancer and the number of sexual partners a woman has had; 2)
the link between liver cancer and alcoholism; and 3) the link between AIDS and the number
of homosexual partners a man has had.
RESEARCHERS FIND NO EVIDENCE OF RECALL BIAS
Dr. Brind Claims Swedish Researchers Covered-Up ABC Link
Recall bias, however, has not been shown to exist by any researchers
in abortion-breast cancer research. In the only study claiming to find
direct evidence of it, Dr. Olaf Meirik found himself in the preposterous position
of having to explain why seven breast cancer patients reported they’d had abortions
which the computer said they’d never had. In other words, 27% (7 out of 26) of the
patients in this group (the group which said they had had any abortions) allegedly lied
and overreported their abortion histories, an assertion which is difficult to believe. The
study was funded by Family Health International. Its authors hypothesized that, “...a
woman who had recently been given a diagnosis of a malignant disease, contemplating causes
of her illness, would remember and report an induced abortion more consistently than would
a healthy control.” [1, p. 1003]
Dr. Chris Kahlenborn, who authored the book, Breast Cancer, Its Link
to Abortion and the Birth Control Pill, criticized the Meirik hypothesis.
He asked, “Why was this the working hypothesis instead of its direct
counterpart?” He suggested a more appropriate hypothesis and argued, “....why did these
authors not originally hypothesize that a woman who has breast cancer might be less candid
about her recall of abortion? After all, ‘denial’ is one of the first reactions that
patients have. When a woman is told that she has breast cancer it is not uncommon to deny
to herself that she really has it. It would seem just as logical to think that such
women would be more likely to deny factors that may have contributed to the
breast cancer such as abortion and/or early oral contraceptive use.” [2, p.
62, emphasis original]
Dr. Joel Brind’s team was also highly critical of the Meirik group’s
claim of overreporting. In a letter published in 1998 in the Journal of
Epidemiology and Community Health, Brind identified serious errors in
Meirik’s study, and proved that that group had covered-up an abortion-breast cancer link in
Norwegian women. [3] To read Dr. Brind’s commentary about the recall bias hypothesis, see his
lecture on our “ABC Link” page.
No Direct Evidence of Recall Bias Among Greek
Women
Others tested for reporting bias and found no direct evidence of it. [4, 5, 6,
7, 8] Lipworth’s group addressed recall bias among Greek women by completing a literature
review. They determined that even before legalization of abortion in Greece there was no social
stigma against it, and it was practiced extensively. Lipworth’s team theorized that a
control group of healthy Greek women would be more likely to be honest about
their reproductive histories. The team reported a 51% increased risk of breast cancer
among Greek women. [4]
Janet Daling’s Studies Report Lack of Response
Bias
Janet Daling tested for recall bias in her 1994 study, the only published
study specifically commissioned by the National Cancer Institute. She found an overall 50%
increased breast cancer risk among women who reported having had an abortion. Daling completed
a separate study to test for recall bias in the study population. She chose another disease
which was known not to be linked to induced abortion -- cervical cancer. She hypothesized that,
if recall bias existed, then patients with cervical cancer would be more likely to report
having had an abortion than healthy controls; and an apparent elevation in risk would
result. If, however, recall bias does not exist in the study population, then an elevation
in risk would not appear.
Using the methods that were employed in the breast cancer study to
identify subjects in the same geographic area, Daling’s results reported no elevation in
risk of cervical cancer from induced abortion. Therefore, she concluded that her methods
were not influenced by recall bias. [5]
The Daling group conducted another test for response bias in a study
reported in the American Journal of Epidemiology in 2000. Researchers collected birth
records in Washington. The records contained reproductive histories furnished by the
mothers at the time of the most recent childbirth, and these were compared to the
reproductive histories reported by the subjects during the study interview.
Daling showed that both patients (cases) and healthy women (controls)
were underreporting their histories of induced abortion at the birth of their last child.
Slightly less than 60% of the subjects in both groups, who said at the time of their
interviews that they’d had an abortion, had also
reported the abortion at the birth of their last child. More importantly,
Daling demonstrated that there was no difference in underreporting level between cases and
controls at study interview. She reported that 14.0% of the cases and 14.9% of the
controls said during their interviews that they had not had abortions, which they had
reported at the birth of their children. No difference in the level of underreporting
means that there was no response bias between breast cancer patients and controls.
Daling said that her findings are compatible with the findings
reported
in two other studies examining reporting bias. [7,8]
PROSPECTIVE STUDY REPORTS INCREASED RISK
A prospective study (i.e. a study in which even the possibility of response bias is non-existent) completed by Howe in 1988 relying on fetal death
certificates filed in New York State at the time of abortion reported a statistically
significant increased risk of 90%. [9]
REFERENCES
[1] Lindefors-Harris BM, Eklund G, et al., Response bias in a case-control study: analysis
utilizing comparative data concerning legal abortions from two independent Swedish
studies. Am J Epidemiol. 1991; 134: 1003-1008.
[2] Chris Kahlenborn, M.D., Breast Cancer: Its Link to Abortion and the
Birth Control Pill, One More Soul, 2000, p. 62.
[3] Brind et al, J Epidemiol Community Health, 1998, 52:209-11.
[4] Lipworth L, Katsouyanni K, Ekborn A, Michels KB, Trichopoulos D, Abortion and the risk
of breast cancer: a case-control study in Greece, Int J Cancer, 1995: 61:181-4.
[5] Janet R. Daling et al., Risk of Breast Cancer Among Young Women:
Relationship to Induced Abortion, 86 Journal of the National Cancer Institute; (1994);1584.
[6] Tang et al, Case-control differences in the reliability of reporting a
history of induced abortion, Am J Epidemiol, 2000, 151:1139-43.
[7] Holt et al, American Journal of Public Health, 1989, 79:1234-8.
[8] Werler et al, American Journal of Epidemiology, 1989, 129:415-21.
[9] Howe et al., International Journal of Epidemiology, 1989, 18:300-4.
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