I asked a bunch of my friends to record their milk consumption and purchasing habits for all of January. Here are the results.
Type
As expected, full-fat milk is the most popular, making up 43% of all milk consumed over the sample period. Skim milk made up a further 34% of milk consumption. Soy (13%) and reduced-fat milk (7.5%) made up almost all of the remainder. Ryan used half and half a couple of times because he is gross.
Meals
Perhaps not too surprisingly, by far the most popular uses of milk were as a snack, or with breakfast. These two meals accounted for 79% of all milk usage. In contrast, comparatively little milk was consumed for lunch, dessert or non-meal uses like baking. This was very different to my experiences, where these plus snacks account for almost all of my milk usage.
Date
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The total volume of milk consumed was the highest in early January (at around 2.5 litres [85 oz] per day), but dropped off slightly to around 2 litres [68 oz] per day towards the middle of January as everyone flaked out. The amount consumed seems relatively consistent from day-to-day, while the amount purchased has a very lumpy distribution. This reflects the consistency of both peoples’ milk usage, but also their regular shopping habits.
Taking a look at the total amount purchased and consumed per weekday, it becomes obvious that most people shop for milk on a Sunday and/or a Wednesday. In addition, consumption is relatively consistent during the week, with a noticeable drop-off of approximately 15-20% less consumption on the weekend.
Time
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Consistent with the data on mealtimes accompanying the milk, the most common time to consume milk is from 8am-9am. This seems a little late to me, shouldn’t you lot be at work? Milk consumption then decreases with each successive hour to minimal usage from 12pm-3pm. However, from then on, it is clearly afternoon snack time. There doesn’t seem to be any uniform time that people take their afternoon snack, dinner, or dessert, but there is a spike at 10pm-11pm, suggesting that people like some milk in their tummies before bed.
The available data on milk purchases (which is a minority – most people didn’t bother noting what time they bought milk) indicates that most people buy their milk early in the morning or the middle of the day. This is likely to be buying things on the way from work or on lunch breaks, unless you are all old people that love to shop first thing in the morning on the weekend.
Milk Purchased and Consumed
The average quantity of container purchased was 2.2 litres [74 oz], and the average quantity consumed at once was 293 millilitres [10 oz]. The most popular quantity of milk to purchase was the half-gallon [1.9 l], which was purchased on 41% of occasions. Following this was the gallon [3.8 l], purchased on 32% of occasions, and the quarter-gallon [0.9 l], purchased on 10% of occasions. You guys sure do like buying huge cartons of milk!
The quantity of milk consumed in one sitting was obviously far more variable. While the distribution was wider, it seems to be bimodal, with the most popular amount to have at once being around 200 ml [7 oz] (mostly due to Jo consuming the same drink two to three times per day), and the second-most popular quantity being around 350 ml [12 oz] (Wes and Ryan’s preferred quantity). Quantities ranged from Gretchen splashing 30 ml [1 oz] of milk into a cup of tea, to Peter and Matt W drowning their troubles in almost a litre of milk (34 oz)! How do you even get glasses big enough to fit that much?
Milk Purchased and Consumed, by Person
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Here is where the flaws of the data are most noticeable. On several people’s returns there are bunches of dates that are close together with a lot of entries (usually towards the start of the month), and then large amounts of dates with no entries at all. There are two possible interpretations to this: 1) people tended to start recording data diligently, and then gave up; or 2) people just don’t use that much milk. Given that this is the internet, I would put a lot of my money on the first explanation. However, the second is almost certainly also true to some extent, and we can’t really say to what extent either is true for individual people.
To resolve this, I decided to calculate milk usage in two ways. The first was the total milk usage you reported in the sheet, divided by 31 days in January. (Some people went above and beyond and recorded milk usage for more than 31 days; for these people, I calculated their milk usage using the first and last dates they entered.) This is your average daily milk usage, assuming you fully recorded every instance of a purchase or consumption of milk. It is represented by the blue bar. The second alternative measure of milk consumption is the average daily milk usage for only the range reported in your spreadsheets. For example, if someone consumed 1,200 ml from 17-19 January, and had no other entries, I took their consumption to be 400 ml per day. This measure applies if everyone stopped filling in their spreadsheet at some point, but continued with exactly the same pattern of consumption. It is represented by the blue bar added to the red bar. It is clear that each person’s true usage of milk in January/February should be somewhere in the range of the red bar.
It is at this point that we can say something about the likely relative flakiness of individuals. Peter, Ryan, Kristen, Wes, and Gretchen all have very small or non-existent red bars, indicating that they kept up the recording exercise throughout the entire period. In contrast, Casey, Davin, and Matt W scored relatively high on this proxy measure of flakiness. You might want to consider what sort of family you married into here, Gretchen.
One of the major reasons for doing this exercise was to see exactly how much milk we as a group tend to consume. From the collected data, we can see that there is a large variation between individuals, with some (such as Peter, Ryan, and possibly Jo and Matt W) consuming upwards of 500 ml [17 oz] per day, while others (Mike, Shara, Amy Berg, Laura, and maybe Casey) would struggle to finish that amount in an entire month. Overall, we consume (at most) about 200 ml [7 oz] of milk per day, which translates to about 73 litres [19 gal] per person per year. Using figures available here, we consume far less than the typical Scandinavian, Britisher, Australian, or Canadian. We consume slightly (about 10-20%) less than the average New Zealander or American. Interestingly, if we all drank as much milk as Peter, we would only be consuming slightly (about 10%) more than the average Finnish person. That’s a lot of milk!!
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We can also compare the reported purchases and consumption, and see how consistent they are with each other. To calculate this measure of consistency, I divided the average daily quantity of milk purchased by the average daily quantity of milk consumed. The resulting ratio gives an average number of litres of milk purchased for every one litre consumed. We can say that people with ratios well above one tend to be very wasteful (allowing your girlfriend to buy a lot of milk but not consuming it all), share their milk with others, or tend to underestimate the amount of milk they use. People with ratios well below one likely tend to overestimate the amount of milk they use, aren’t the household purchasers of milk (would it kill you to do the shopping once in a while Casey, Laura, and Matt W), or simply steal it off other people in the office (looking in Wes’s direction).
Posted by penix 
