How much weight do you need to lose to drop an inch off the waist?

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This is an analysis inspired by Dan’s 2014 article and it’s meant to be a continuation of his idea with a larger sample size and a slightly different approach. 

I’ve used the PRAW package in Python to scrape data from the progresspics subbredit, and I ran the data analysis in R. I was able to collect data from 77 individuals and I chose to split the entire analysis by gender – due to the differences in abdominal fat distribution between men and women.
As can be observed from Table 1 and the plot, weight loss tends to be much higher for the males than for the females – both in absolute units and percentage-wise. 

I chose to run four different regression models, for each gender. While Dan’s article only considered weight change, I also included the weight percentage change, BMI change, and BMI percentage change.

# Male
lm(weightDiff ~ waistDiff, data, subset = gender == "Male")
lm(weightDiff_perc ~ waistDiff, data, subset = gender == "Male")
lm(bmiDiff ~ waistDiff, data, subset = gender == "Male")
lm(bmiDiff_perc ~ waistDiff, data, subset = gender == "Male")

# Female
lm(weightDiff ~ waistDiff, data, subset = gender == "Female")
lm(weightDiff_perc ~ waistDiff, data, subset = gender == "Female")
lm(bmiDiff ~ waistDiff, data, subset = gender == "Female")
lm(bmiDiff_perc ~ waistDiff, data, subset = gender == "Female")

Based on the results from Table 2, we could conclude the following:

Males

1. On average, you need to lose 8.6 lbs (3.9 kg) to lose 1 inch off the waist.
2. On average, you need to lose 1.4% of your weight to lose 1 inch off the waist.
3. On average, you need to lose 1.1 BMI points to lose 1 inch off the waist.
4. On average, you need to reduce your BMI by 1.4% to lose 1 inch off the waist.

Females

1. On average, you need to lose 4.8 lbs (2.1 kg) to lose 1 inch off the waist.
2. On average, you need to lose 2.3% of your weight to lose 1 inch off the waist.
3. On average, you need to lose 0.82 BMI points to lose 1 inch off the waist.
4. On average, you need to reduce your BMI by 2.4% to lose 1 inch off the waist.

Of course, the sample size is relatively small (especially for women) and the starting weights vary widely, so take these results with a grain of salt.
If you  want to delve deeper, you can find the data and the code I used here.