by Ginny Knackmuhs, MBCN Vice President
Recent commercials for the Komen 3 day walk make this claim: a 30% drop in breast cancer mortality rates since the early 90’s due to early detection. Is this true?
According to Wikipedia, mortality rate is a measure of the number of deaths in a population, scaled to the size of that population, per unit of time. Mortality rate is typically expressed in units of deaths per 1000 individuals per year. So a breast cancer mortality rate of 31.6 in 1992, for example, means there were approximately 31 deaths for every 1000 individuals living with breast cancer at that time.
Statistics can be very confusing. When I first saw this claim of a 30% drop in mortality back in 2011, I was shocked and sure that Komen was wrong. Did this mean that 30% fewer people were dying from breast cancer? That sounded too optimistic. I wrote to my friend who has a masters in applied math and economics and she enlightened me:
The mortality rate in 1992 was 31.60 and on average has dropped 2.2% every year. In 2008, for example the mortality rate was 22.14. Over that 17 year period, the mortality rate dropped by 9.46. (31.6 in 1992 minus 22.14in 2008= 9.46) See charts below:
So what about the 30%? Shouldn’t it be 9.46%? Ahh…welcome to the mysterious world of statistics.
The % change in the mortality rate that Komen is quoting is calculated differently. It is a % of a % calculation. The actual rate change of -9.46% is divided by the original rate of 31.6%, giving a % change in mortality rate over 17 years of (-9.46/31.6) or 29.9% change. This is the figure that Komen is referencing. (30% drop in BC death RATES over a given period of time)
Here’s a simpler example. If we raised the tax rate from 5% to 10%, we have doubled the rate (100% increase because 5% change divided by the original 5%rate = 100 —also look at it this way: 10 is twice 5, a 100% increase) although the actual rate change is an additional 5%. If you were the angry taxpayer, you would probably report this as a 100% increase in your tax rate, rather than saying it was an addition of 5%. Both are accurate statistically, but you can see the potential for spinning the numbers to support your campaign.
I read another example in the newspaper this morning. The headline said: “ADHD cases up 24% in a decade.” I immediately envisioned almost a quarter more of ADHD children, the same way I envisioned 30% fewer people dying from breast cancer. Neither is true. The ADHD article went on to explain the basis of the 24%: “The disorder was detected in 3.1 percent of children who received care at Kaiser Permanente Southern California in 2010, compared with 2.5 percent in 2001, according to the study, published Monday [1/20/13] by the journal JAMA Pediatrics.” In other words, 2.5% rose to 3.1 %, an additional .6%, but compared to the initial rate, a 24% change in rates (.6%/2.5%= 24%)
I told you it’s confusing!! You can usually find some statistic to make your position look good. That being said, in the metastatic bc community, we usually focus on the NUMBER of people dying and not the RATES. Despite early detection and advances in treatment, the actual number of deaths over the last 20 years has remained essentially unchanged, hovering around 40,000 deaths per year.
NBCC explains it this way:
Truth #5: Breast cancer mortality rates are declining
TRUE. Breast cancer death rates changed little between 1930 and 1990, but decreased 27% from 1990 to 2005.
Between 1994 and 2003, the mortality rate for women of all races combined declined by 2.4% annually. In white women, breast cancer mortality declined by 2.5% annually. In black women, mortality declined by 1.4% annually during the same period. Some good news, but remember there are still 40,000 women who will die of breast cancer this year.
While these statistics are encouraging — we don’t actually know why mortality rates decreased. We need more research to figure out what factors led to the reduction in death from breast cancer so that we may continue the downward trend — and we need to make certain it’s the same for all women. We must continue pushing to find out what causes this disease.
Komen attributes all of the change to early detection. Certainly changes in treatment have also helped. The American Cancer Society 2011-2012 report states: “The decline in breast cancer mortality has been attributed to both improvements in breast cancer treatment and early detection.”
What’s the lesson here for those of us living with metastatic breast cancer? I hope after this little math lesson that you are forever grateful that you don’t have to sit through Statistics 101! My friend the economist who explained all this to me, ended her email, saying: “Hope this helps. Anyway, it would make me much happier if the mortality rate were 31.6 – 29.9!”
I hope you understand better the statistics being quoted and will read articles closely in the future, where the same type of statistic is used. While legitimate statistically, the percent change in mortality rates hides the reality of a hard core of stage IV patients who continue to die. After forty years of research in the War on Cancer, we still don’t know what causes breast cancer or what causes it to spread. We need to continue to advocate and raise awareness of metastatic disease and the lack of progress in preventing and stopping metastases.