Mathematically, checking more frequently will just decrease the amount of clock time necessary before you've gathered enough samples for your result to be statistically significant. In other words, the thing that's important isn't how frequently you check, but the absolute number of times that you've checked. So in a given day, you will be able to gather 1440 samples while the OP will only have been able to gather 48 samples. If you're interested, we could go through an exercise of calculating the margin for error on the uptime statistics over various time periods using both 1-minute and 30-minute sampling at various uptime levels.
Again, sampling every 30 minutes just means that it will take the OP 30 times as long to get as good a baseline as you, given that you're sampling every minute.
(Though actually, this is a worst case because it actually assumes that outage events are instantaneous. If outage events last an hour, sampling every minute and sampling every 30 minutes will actually require a similar number of calendar days before reaching statistical significance.)
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