I think the overwhelming majority of people are visiting grocery stores to buy food. (Or have someone in their household doing it)

If the studied presume this then it makes more sense.

You can't assume that. I know plenty of people who haven't left their apartments in a month plus, because they're having groceries ordered in.

And you completely missed the point that people who are going outside a lot more often are both (a) more likely to have been exposed to COVID-19, and (b) more likely to be encountered by your survey. If there are e.g. 700 daily shoppers and 700 weekly shoppers going to a given store, then if you go on one day and sample every shopper you're not going to get half and half by population as you'd need to for a true random sample; instead, you're going to get 700 daily shoppers and only 100 weekly shoppers! This is a hugely biased sample!

The study wasn't one day but one week, and even then, I assume they wouldn't be stupid enough to collect blood from the same person over and over.

There's more than one store in NYC. People shop at a variety of places. Daily shoppers are likely not going to the same store over and over every day. Even during this pandemic I've hit up several different grocery stores and shops, because some things are hard to find.

Are you describing the actual study or the study as you're imagining it? Because I imagine the statisticians have thought of these concerns and attempted to correct for them.

I find it interesting that this study corresponds to an IFR around 0.5%, similar to the IFR arrived at by other studies using very different methods. Perhaps the statisticians who conduct these studies actually know what they're doing better than Hacker News commenters.

And perhaps it's time to accept the politically inconvenient scientific truth these studies are revealing.

>similar to the IFR arrived at by other studies using very different methods

With the wild range of IFR from so many studies, you could trivially cherry pick supporting studies for a bunch of different numbers, especially if you throw in am error range.

I’m starting to believe that the difference in opinions on whether these studies are legitimate or not hinges heavily on personal behavior.

For my wife and I and kid, we live in SF and order food when we can from grocery. When we can’t, I put on an N99 mask and gloves and buy 2 weeks worth of groceries. Then go through a ritual to be as safe as possible entering the home and bringing groceries in.

If someone asks me at the grocery store to take the covid test, no way. How would I even do that safely???

I live in San Francisco though and am worried about the fact that the grocery is 70% instacart workers.

Everyone I know in the city (nyc, sf, Boston, Chicago etc.) is trying to use 3rd parties to get their groceries.

Everyone I know in the suburbs is going to the store.

I believe the reason people read this study differently is that their experience is different. The whole foods in downtown sf would be scary to be in without a mask. And I view it as the riskiest place I have to go.

It’s full of gig workers who are taking way more risk than the average person.

We will know more when we see the results of the study, but I think we need a more random sample before thinking this NYC study is even remotely close to showing that 20% of NYCers previously had it

I'm afraid the population who has these options (and spends its time on hackernews) is called the 1% for a reason, and probably aren't going to bias the stats significantly.

This doesn't make any sense. Grocery and food delivery is quite affordable. It's not only available to the top 1%. These options are being widely taken advantage of by a large number of people right now way down the economic spectrum. It's not a tiny minority of the wealthiest people patronizing them.

At any given point of time, a disproportionate number of shoppers in a store are super-shoppers - people that shop more often than the average person and for longer periods of time. Those people are also more likely to have caught Coronavirus before. Without even getting into online ordering, false positives, etc...this phenomenon alone may confound the test results.

How do you know how often these people are leaving their homes? Everyone has to leave their home to go to the grocery store eventually or someone in their family does. Yes there are delivery services but the vast majority of people aren't using those.

The more often they go, the more likely you encounter them.

True, but if you encounter one daily shopper, two weekly shoppers and twenty monthly shoppers, it will skew the results. Same if the numbers are reversed.

But you won't, the opposite will happen; you'll proportionally encounter way more daily shoppers than infrequent shoppers, for the simple tautological reason that the daily shoppers are going more often.

This is a sampling bias, and it's the exact same problem as if you ran a survey trying to ascertain how many people have landlines ... by calling random phone numbers.

I don’t get your point. Maybe we agree. If in one city there are 1000 30-year-old daily-shopers, 1000 50-year-old weekly-shoppers and 1000 70-year-old monthly-shoppers it will not be a good idea to estimate the age of the population by sampling people who enters the store. The average age is 50, not 34.

Most people in NYC are almost certainly not shopping monthly because of storage constraints. Even bi-weekly shopping is likely to exhaust storage space for city-dwellers in small apartments.

Couldn't you get a bad study that reverses the sampling bias to get a decent picture of social distancing strategies using the two bad studies? That alone seems far more useful than knowing the IFR because its actionable information.

Or better yet, do a simple survey and ask the participants how frequently they go to the grocery store. And if you see a correlation between frequency and infection rate, then try and adjust your weights accordingly.

These bias issues are well known among survey designers. There are ways that you can try and account for them. It seems like maybe the medical community is too used to having control groups in their studies?

Most people go to the grocery store. There seems to be this attitude on HN of immediately dismissing research. No human research is ever perfect, flaws and limitations will always be present, and in the present case, study designs are being expedited in order to be timely. Each study is a piece, and together can provide convergent evidence.