It's 2021! Time for a crash course in four terms that I often see mixed up when people talk about testing: sensitivity, specificity, positive predictive value, negative predictive value.
These terms help us talk about how accurate a test is, but from different viewpoints. 1/
A test that is very *specific* will be very good at accurately ruling out infection in people who are not infected. 3/
Of the 10 people infected, 8 test + (true +), 2 test - (false -).
Of the 90 people uninfected, 89 test - (true -), 1 tests + (false +). 7/

The test's positive predictive value is true positives/(true positives + false positives): 8/9, or 88.9%. It's the proportion of positives, out of all the positives, that were accurate. 10/
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New from me:
I’m launching my Forecasting For SEO course next month.
It’s everything I’ve learned, tried and tested about SEO forecasting.
The course: https://t.co/bovuIns9OZ
Following along 👇
Why forecasting?
Last year I launched https://t.co/I6osuvrGAK to provide reliable forecasts to SEO teams.
It went crazy.
I also noticed an appetite for learning more about forecasting and reached out on Twitter to gauge interest:
The interest encouraged me to make a start...
I’ve also been inspired by what others are doing: @tom_hirst, @dvassallo and @azarchick 👏👏
And their guts to be build so openly in public.
So here goes it...
In the last 2 years I’ve only written 3 blog posts on my site.
- Probabilistic thinking in SEO
- Rethinking technical SEO audits
- How to deliver better SEO strategies.
I only write when I feel like I’ve got something to say.
With forecasting, I’ve got something to say. 💭
There are mixed feelings about forecasting in the SEO industry.
Uncertainty is everywhere. Algorithm updates impacting rankings, economic challenges impacting demand.
It’s difficult. 😩
I’m launching my Forecasting For SEO course next month.
It’s everything I’ve learned, tried and tested about SEO forecasting.
The course: https://t.co/bovuIns9OZ
Following along 👇
Why forecasting?
Last year I launched https://t.co/I6osuvrGAK to provide reliable forecasts to SEO teams.
It went crazy.
I also noticed an appetite for learning more about forecasting and reached out on Twitter to gauge interest:
The interest encouraged me to make a start...
I’ve also been inspired by what others are doing: @tom_hirst, @dvassallo and @azarchick 👏👏
And their guts to be build so openly in public.
So here goes it...
In the last 2 years I’ve only written 3 blog posts on my site.
- Probabilistic thinking in SEO
- Rethinking technical SEO audits
- How to deliver better SEO strategies.
I only write when I feel like I’ve got something to say.
With forecasting, I’ve got something to say. 💭
There are mixed feelings about forecasting in the SEO industry.
Uncertainty is everywhere. Algorithm updates impacting rankings, economic challenges impacting demand.
It’s difficult. 😩