MSM in Hong Kong
MSM does not mean MainStream Media in
this post. Instead, it means Men Having Sex With Men.
The starting point is this Ming Pao ad taken
out by the Truth & Light Society. I found this from Miss
Two sets of numbers are cited in this
The first set of numbers is derived from Lau,
J.T., Lau, M. and Tsui, H.Y. HIV related behaviors and attitudes among
Chinese men who have sex with men in Hong Kong: a population-based on
study. Sex. Transm. Infect., 2004 Dec: 80(6), 459-465. The data are
summarized in the schematic diagram
below. Please be mindful that I have not read the original paper, and do
not know if there are more complex sampling issues involved here. I only
have this diagram to go by.
The second set of numbers comes from a press
release from the Virtual AIDS Office of Hong Kong. The data are as
A total of 268 HIV cases were reported to the
Department of Health (DH) last year. ... Of these cases, 107 involved heterosexual transmission and 65 occurred between men having sex with men.
"Under the voluntary reporting system, some 28 per cent of the reports were incomplete, making the determination of the route of infection impossible,'' Dr Lee said.
Now I will translate for you what the ad
A survey conducted in Hong Kong estimates
that 4.6% of men had engaged in MSM. In the past 6 months, about 2% had
engaged in MSM.
According to the information provide by
the government, during 2004 there were 172 cases of new HIV cases that
occurred as a result of sexual contact. Of these, 65 were infected via
In other words, the male homosexuals
account for only 2% of the population accounts for 37.7% of the new HIV cases
in 2004. Based upon these figures, the odds ratio of MSM to heterosexual
behavior in causing HIV infection is 29:1.
Let us do these numbers line by line.
- There were 14,985 in the LLT study, of whom
667 ever had MSM. Thus, the percentage is 100 x 667 / 14985 =
4.45%. (So maybe this is not the 4.6% listed, but so what?)
[Alternately, a reading suggested that the 4.6% came from J T F Lau, J H Kim and H-Y
Tsui (2005) Prevalence and factors of sexual problems in Chinese males and females having sex with the same-sex partner in Hong Kong: a population-based study,
International Journal of Impotence Research (see abstract).
This was cross-sectional, anonymous telephone survey. Would you
discuss your sexual preferences and practices with a stranger over the
- There were 14,985 in the LLT study, of whom
292 engaged in MSM. Thus, the percentage is 100 x 292 / 14,985 =
1.95%. (This is the 2% listed)
- There were 65 cases of MSM HIV cases in the
- Where did the 37.7% come from? As far
as I can see, 100 x 65 / (107 + 65) = 100 x 65 / 172 = 37.8%. But what
is it? 65 is the number of HIV infections via MSM whereas 172 is the
number of HIV infections via sexual contact, both heterosexual and
MSM. Properly speaking, this is defined as the percent of MSM HIV
infections out of all sexually transmitted HIV infections.
- Now we parse this most astonishing
statement: the male homosexuals account for only 2% of the population but
also account for 37.7% of the new HIV cases in 2004.
This is wrong in several ways.
If you want to preserve the numbers, you have to change the language as
follows: the male homosexuals account for only 2% of the population but
alsl account for 37.7% of the new sexually transmitted HIV cases in 2004.
If you want to save the language, you will have to change the number.
There were 268 cases in the government's records, but some 28% have no route
of infection. Thus, the number of classificable cases = 268 x 0.72 =
193. Of these, 65 were infected via MSM behavior. 100 x 65 / 193
= 33.7%. Thus, the statement becomes: the male homosexuals account
for only 2% of the population but also account for 33.7% of the new classifiable HIV
cases in 2004.
But now we hit the biggest stumbling block in this
What is the universe of the LLT study? The title is "HIV related
behaviors and attitudes among Chinese men ..." So, the universe
and the study sample are "Chinese men" (assuming that means males 18
years or older of Chinese descent).
What is the universe for the government's records? You can read
through the press release and you won't find any mention about Chinese
men. That universe is the total population. Who is in the total
population but not a Chinese man? Babies who can receive perinatal infections,
children under 18, the entire female population, European men, Southeast
Asian men, Japanese men, and so on. Within the 65 MSM HIV cases, there may be minors
and non-Chinese men. Within the 107 HIV cases involving heterosexual
transmission, there also women, minors and non-Chinese men. Within
the 268 total HIV cases, there are infants, children, women and non-Chinese
So what is the ratio of MSM HIV infections among all HIV infections within
Chinese men? I don't have a clue. The government data here do
not say so. If you want to know, you'll have to go back and get that
information from the government. But I do know that the assertion in
the ad is WRONG because the two parts have different universes.
The actual number may be
more, the same or less than 37.7%. But the point here is that you
cannot assert 37.7% on the basis of these two datasets. You can assert
it on the basis of personal belief or faith and that is your
prerogative. But you cannot say, "The data show ..."
The viability of the science of statistics depends on this.
The Truth & Light Society ad was signed by a
whole lot of healthcare and medical professionals who placed their names onto a
petition based upon scientific data and arguments. The data may be
scientific but the statistical inference is wrong. And this is not exactly
rocket science because the survey universes are clearly different. These people must have put their signatures down without
even looking at the statements. Think about that kind of professionalism
the next time that you visit a doctor.
Since this is paid advertisement and the
information is wrong, it is false advertising. Both the advertiser and the
newspaper have liabilities as such.
Message to MSM (mainstream media in Hong Kong):
This is scandalous. Will you pick it up and report it?