I have watched different videos on youtube where they are explaining the audience that admob #admob pays money for impressions.They are saying this based on its defination.

// Does setting floor for ecpm increases your income ? //
No, it depends on the geographical area where your app has been mostly downloaded , advertisers and more . Ecpm is the factor with the help of which you are trying to allow ads that can earn you the minimum amount i.e , ecpm floor . But, if there is no ad or less ads that can earn you the minimum (ecpm floor) from the place where your app has most active user's then, as a result your fill rate will decrease and blank ads will be displayed ; therefore income will be decreased .
Hence, ecpm can increase or decrease your income. And , you have to be careful while setting the ecpm value to increase your income .You can however get to know about the best value by experimenting with different number's and analysing in the report section for the best value .You can set different ecpm floor for different countrie's in the ad unit settings section.
You can set so many ecpm floor for the same ad unit in the mediation section and set the priority for the different mediation groups that you have created in the descending order of the floor and it is the best way because your ad unit will be filled with the ad that pays you the maximum amount .
SO , IN ORDER TO EARN THE BEST FROM ADMOB YOU SHOULD CREATE DIFFERENT MEDIATION GROUPS , SET PREFERENCES TO THEM AND ANALYSE YOUR AD PERFORMANCE IN THE REPORT SECTION.

Watch the complete video to understand ecpm properly.#whatisecpm
admob help : https://support.google.com/admob/answer/3016008?hl=en
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https://www.youtube.com/watch?v=OyZaA5bJFns