I'm fairly sure (but not entirely positive, maybe someone who knows for sure will jump in here) that the tech rating is based on how many techs you have researched. Not the expense or relative value, just straight number of techs. So for a quick boost you can research a bunch of low level defensive techs, where you can get several techs per week (on normal research speed, that is). Probably won't help you much in a practical sense, but it will sure look impressive   

I always believed it was based on your total RP output. I have a bad habit of jumping around the cheap techs, researching them quickly(not the most efficient way to research) and that in itself doesn't seem to help the way your ranked on tech. In fact just checking my game and looking at the RP of each AI and myself it does seem directly related to your RP output.

I always check the stats and graphs section since that little popup is really misleading. You can be producing outproducing the AIs at a certain point, but the popup I think gives general overall from throughout the game. So if your say outproducing the AI through three quarters of the game and then drop your research spending you'll stay ranked number one until one of the AIs catch you. I do wonder if certain tech does give more value than others, but have really seen nothing that shows it if it does.

Hi everyone:) Thanks for replying.Reading through all the posts, I guess I neednt get too distressed about the Tech rating? I am a rank newbie and I just research a few cheap techs at first and then go for something more time consuming.

I am pretty sure that if you build more Research Centres on your planet, which will increase your research efficiency, you can research more techs and as said above, the combination of this will increase your Research rate, but you shouldn't be worried about it anyway, just make your way in the galaxy and conquer everything you see, it gives most points.