Unable to upgrade r-base-core package to backports in Debian

Question

I am trying to upgrade r-base-core from version 3.1.1-1 in Debian Jessie to version 3.3.2-1 in jessie-backports. My setup:

$ apt-cache policy r-base-core
r-base-core:
  Installed: 3.1.1-1+deb8u1
  Candidate: 3.1.1-1+deb8u1
  Version table:
     3.3.3-1~jessiecran.0 0
        500 http://cran.cnr.berkeley.edu/bin/linux/debian/ jessie-cran3/ Packages
     3.3.2-1~bpo8+1 0
        100 http://ftp.debian.org/debian/ jessie-backports/main amd64 Packages
 *** 3.1.1-1+deb8u1 0
       1000 http://security.debian.org/ jessie/updates/main amd64 Packages
       1000 http://security.debian.org/ stable/updates/main amd64 Packages
        100 /var/lib/dpkg/status
     3.1.1-1+b2 0
        900 http://ftp.us.debian.org/debian/ jessie/main amd64 Packages
        900 http://mirrors.kernel.org/debian/ stable/main amd64 Packages
        900 http://ftp.us.debian.org/debian/ stable/main amd64 Packages

When I execute sudo apt-get -t jessie-backports install r-base-core , I get:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
r-base-core is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and 205 not upgraded.

Why doesn't it see the jessie-backports version as being newer? How do I debug this? Thanks for any help.

Answer 1

The display shows the *** for the selected variant. Below is the all-important score of a 1000 which is higher than the alternatives of 100 and 500, respectively, for the higher versions.

This is a feature called apt-pinning. You need to adjust the ranking you give the different repos. At equal or lower score , the upgrade to a higher version happens as you would expect.