Methods
Participants
The data used in this study (Appendix A and Supplementary Material File)
was collected by the means of crowdsourcing. With the intention of
avoiding ethical issues, question items on blood type characteristics of
this study were extracted from reviewed academic articles (Sato et al.
1992; Watanabe 1994). These items were checked again at the previously
stated crowdsourcing company which has been passed the Japanese privacy
mark (JIS Q 15001). The company confirmed that there was no problem with
the question items. It provides customers with anonymized data and
obtains informed consent from participants (respondents) prior to its
surveys.
Survey 1 was conducted in 2018 as a preliminary study with a sample size
of 1,000. Japanese single individuals with the age of 20-39 were asked
to rate a total of 8 items representing 4 blood type (A, B, O, AB)
characteristics, each with scores of 1-5 for their personality traits
(the larger the number, the more fitting the trait), and scores of 1 to
4 of relationship and their knowledge on the level between blood type
and personality (the larger the number, the more related or informed).
Survey 2 was conducted in 2019, and covered 2,000 Japanese people ages
20-59, further expanded blood type characteristics to 12 items, and
asked the respondents to answer which blood type they thought these 12
items would categorize. In Survey 2, we added an item of marital status,
since the age of participants spanned to 59, which meant that married
persons were included.
Each participant’s blood type of was determined by self-report, because
most Japanese people know their blood type. In Survey 1, the dataset
included only those who knew their blood type. Hence, the valid sample
size was 1,000. In Survey 2, 141 out of all the 2,000 respondents, or
7.1% of the total, did not know their blood type. Thereby the data of
the remaining 1,859 participants with known blood types were used.
The result of preliminary Survey 1 suggested that people who were less
interested in blood type had traits of introvert, unsociable, and less
interested in personality. Subsequently we added three new question
items in Survey 2 to confirm this. These items were determined by the
same process that was used to extract question items on the
characteristics of the blood type. All samples were equally allocated by
age of 10-year increments, and gender.
Procedure and Analytical
Strategy
We deliberately selected the most suitable characteristics that would
certainly produce the differences, and then focused our analysis on
whether personality self-fulfillment was occurring or not: with data
from participants who “have no knowledge of blood type
characteristics” (hereinafter abbreviated as “no-knowledge group”).
Survey 1 had only 120 “no-knowledge” participants therefore we would
mainly use the 325 “no-knowledge” participants of Survey 2, who were
free of “self-fulfilling”, because they had no knowledge of blood type
personality. Differences between groups with large and small
characteristic differences according to blood type were also examined.
Our analytical methods were as follows:
Analysis 1: Cross-tabulation of blood type and personality ‒ Survey 1
and 2
Analysis 2: Cross-tabulation of knowledge and personality ‒Survey 1 and
2
Analysis 3: Cross-tabulation of knowledge, extroversion and sociability
‒ Survey 2 only
We set the alpha level to 0.05. The ANOVA, and the binominal test were
used to analyze blood type characteristics. The effect sizes (Cohen,
1977) were also calculated. The dependent variables were analyzed by
both the blood type factor and the covariates of gender, age (10-year
increments), and marital status (since Survey 1 participants were all
single, Survey 2 only). The distributions of blood types were almost
equal to the Japanese average (Okubo, 1997), so the results of
χ2 test for the bias in both surveys were not
significant.