Terry Bellamak: Self-selection is entirely different to purposive sampling

By Terry Bellamak

NZ Herald·

9 May, 2019 02:23 AM3 mins to read

Terry Bellamak, president of the Abortion Law Reform Association of NZ. Photo / Mark Mitchell

COMMENT

An opinion piece by Rachael Wong, entitled "Recognising the end of pregnancy in cases of abortion and miscarriage" (NZ Herald, April 30) contains the following statement:

"Everard maintains that 95 per cent of women who undergo an abortion 'do so without regrets'. However, the study that this statistic is based on is deeply flawed (due to self-selection bias, among other things) and does not reflect the reality of the many women who suffer grief and other psychological issues after an abortion."

Wong's statement that "the study that this statistic is based on is deeply flawed (due to self-selection bias, among other things)" is factually incorrect. Everard said the statistic she referenced came from a post on ALRANZ's blog.

The blog post referred to an article on theconversation.com, which references Decision Rightness and Emotional Responses to Abortion in the United States: A Longitudinal Study, often referred to as the Turnaway study. The description of the study's methods states:

"We recruited a cohort of women seeking abortions between 2008-2010 at 30 facilities across the United States, selected based on having the latest gestational age limit within 150 miles. Two groups of women (n=667) were followed prospectively for three years: women having first-trimester procedures and women terminating pregnancies within two weeks under facilities' gestational age limits at the same facilities. Participants completed semiannual phone surveys to assess whether they felt that having the abortion was the right decision for them; negative emotions (regret, anger, guilt, sadness) about the abortion; and positive emotions (relief, happiness)."

Quantitative research employs different sampling techniques for different purposes. "Self-selection" is one such technique. The sampling technique employed by researchers for the Turnaway study, however, is "purposive sampling", which is different. These general definitions come from the paper Pros and cons of different sampling techniques published in the International Journal of Applied Research.

"Purposive sampling, also known as judgmental, selective or subjective sampling, reflects a group of sampling techniques that rely on the judgement of the researcher when it comes to selecting the units (e.g. people, case/organisations, events, pieces of data) that are to be studied.

"Self-selection sampling is appropriate when we want to allow units or cases, whether individuals or organisations to choose to take part in research on their own accord. The key component is that research subjects volunteer to take part in the research rather than being approached by the researcher directly.

These two things are not the same. It cannot be credibly argued that they are. Nor can the belief that these two things are the same be credibly passed off as a matter of opinion. It is simply incorrect.

We understand anti-abortion groups, who would like nothing better than to pretend abortions are harmful to the mental health of pregnant people, deeply dislike this peer-reviewed, longitudinal study. The study has garnered much media attention and praise.

They are welcome to criticise this study, or anything else, provided the criticism does not misrepresent facts. Everyone is entitled to their own opinions, but not their own facts.