7 ms·

This can't be true. Any method must be at least as good as a coin toss. If it's worse you can just bit flip the result and end up better than the coin.

detrites 4y ago
Unbelievably, the paper actually makes that claim: > That is, if you toss a coin, you have a 50% chance of correctly choosing the value. In many instances, using a MFM to determine the prior value written to the hard drive was less successful than a simple coin toss. More believably, the paper was written by Dr Craig Wright.
- kryps 4y ago
  The paper does not make that claim. According to the paper the probability of recovering the correct value of a single bit is 96% for a pristine drive and 56% for a used drive, both better than a coin toss. Of course the probability gets worse if one tries to recover more bits (but still better than n coin tosses for n bits).
- marginalia_nu 4y ago
  Right, if you flip a hundred coins, about half of them land heads and the other half tails. The correct coins we'll call oracles, and the incorrect ones liars. Now you perform a hdd recovery that is successful 70% of the time. That is still worse than the performance of the oracle coins which were correct, and if you invert the performance of the liars, they also beat the performance of the recovery. If you have two coins, one which is always correct and one which is always incorrect, you can flip one of them to decide whether the other one is correct (and pick the opposite). Check mate, statisticians!
mecsred 4y ago
I don't think that follows? Just because a better method exists (the inverse) doesn't make the original perform better. Imagine the following method: take whatever the bit was, and set it to the opposite of that. 0% < 50%. Of course you can invert and get 100%, but that's a different method. It doesn't make the original any better.
DSMan195276 4y ago
The article isn't super clear, but I think what it's saying is that the data recovery method they were testing (when looking at recovering a single bit) was as good as 92%, and as bad as 32%, depending on how the data was overwritten and other factors like the specific drive. Thus you likely can't just flip the result to get better odds because in a real scenario you wouldn't know which category you fall into (whether it's really good, or really bad), and overall if you don't know that information then the odds are close to 50%. Edit: And of course even a high of 92% is abysmal for a single bit, since the combined odds of multiple bits quickly goes to zero for any sizable amount of data.
kmeisthax 4y ago
Bits are correlated: we care about the order of the bits, not just their population count. Furthermore, <50% is only an antipredictor for a single coin flip. If you have two bits - two coin flips where you care about the order - then there's four values, and you have to be <25% to be an antipredictor. Even then, an antipredictor only removes one possibility[0], so flipping the bits will only slightly improve your terrible chances of recovering the data. [0] https://en.wikipedia.org/wiki/Monty_Hall_problem https://en.wikipedia.org/wiki/Monty_Hall_problem

detrites 4y ago

Unbelievably, the paper actually makes that claim: > That is, if you toss a coin, you have a 50% chance of correctly choosing the value. In many instances, using a MFM to determine the prior value written to the hard drive was less successful than a simple coin toss. More believably, the paper was written by Dr Craig Wright.

kryps 4y ago
The paper does not make that claim. According to the paper the probability of recovering the correct value of a single bit is 96% for a pristine drive and 56% for a used drive, both better than a coin toss. Of course the probability gets worse if one tries to recover more bits (but still better than n coin tosses for n bits).
marginalia_nu 4y ago
Right, if you flip a hundred coins, about half of them land heads and the other half tails. The correct coins we'll call oracles, and the incorrect ones liars. Now you perform a hdd recovery that is successful 70% of the time. That is still worse than the performance of the oracle coins which were correct, and if you invert the performance of the liars, they also beat the performance of the recovery. If you have two coins, one which is always correct and one which is always incorrect, you can flip one of them to decide whether the other one is correct (and pick the opposite). Check mate, statisticians!

mecsred 4y ago

I don't think that follows? Just because a better method exists (the inverse) doesn't make the original perform better. Imagine the following method: take whatever the bit was, and set it to the opposite of that. 0% < 50%. Of course you can invert and get 100%, but that's a different method. It doesn't make the original any better.

DSMan195276 4y ago

The article isn't super clear, but I think what it's saying is that the data recovery method they were testing (when looking at recovering a single bit) was as good as 92%, and as bad as 32%, depending on how the data was overwritten and other factors like the specific drive. Thus you likely can't just flip the result to get better odds because in a real scenario you wouldn't know which category you fall into (whether it's really good, or really bad), and overall if you don't know that information then the odds are close to 50%. Edit: And of course even a high of 92% is abysmal for a single bit, since the combined odds of multiple bits quickly goes to zero for any sizable amount of data.

kmeisthax 4y ago

Bits are correlated: we care about the order of the bits, not just their population count. Furthermore, <50% is only an antipredictor for a single coin flip. If you have two bits - two coin flips where you care about the order - then there's four values, and you have to be <25% to be an antipredictor. Even then, an antipredictor only removes one possibility[0], so flipping the bits will only slightly improve your terrible chances of recovering the data. [0] https://en.wikipedia.org/wiki/Monty_Hall_problem https://en.wikipedia.org/wiki/Monty_Hall_problem