When reading the original papers from Planck Collaboration, I am always confused by the sentence like that ''this measument ([tex]H_0=73.8\pm2.4[/tex] km/s/Mpc) is discrepant with the planck estimate ([tex]H_0=67.3\pm1.2[/tex] km/s/Mpc) at about the 2.5[tex]\sigma[/tex].''
Here, how is the value of 2.5[tex]\sigma[/tex] calculated?
I have tried many ways to get this result, but none of them satisfies me. Can anybody enlighten me?
How to quantify the discrepancy between two observational results with errors?

 Posts: 1723
 Joined: September 23 2004
 Affiliation: University of Sussex
 Contact:
Re: How to quantify the discrepancy between two observationa
If you have two independent measurements of H, H1 and H2, with Gaussian errors, then you'd expect mean(H1H2)=0, with variance \sigma^2=\sigma_{H1}^2 + \sigma_{H2}^2. In this case H1H2 ~ 6.5, \sigma^2~2.68, and hence H1H2 ~ 2.4 sigma.
Re: How to quantify the discrepancy between two observationa
Although this approach give a slightly different value, it is definitely a scientific approach. Now I can decide which way to use. Thanks a lot!Antony Lewis wrote:If you have two independent measurements of H, H1 and H2, with Gaussian errors, then you'd expect mean(H1H2)=0, with variance \sigma^2=\sigma_{H1}^2 + \sigma_{H2}^2. In this case H1H2 ~ 6.5, \sigma^2~2.68, and hence H1H2 ~ 2.4 sigma.