Perhaps you should try some spelling variations:
population center » population census, population censuses, population change
2.2 communication » 2.3 communication, 3.2 communication, 2.7 communication
technology set » technology sector, technology 1, technology 2.2
set population » meet population, de population, em population
4.4 developing » 5.4 developing, 6.4 developing, 1.4 developing
center based » gender based, cluster based, weather based
based 2.2 » bases 2.2, used 2.2
costs 4.4 » costs 4.2

You may be able to get more results by adjusting your search query.

• Adding a wildcard symbol may retrieve word variants: technology set population center based 2.2 communication costs 4.4 developing _ african.*.