For iPhone an Android design, why we set the psd resolution up to 72? why we aren't using 160 for mdpi and 240 for hdpi so on.
Eg: for iPhone 2x res we are choosing 640 x 960px and and screen resolutions 72 pixel/inch . Why this 72 ?
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Setting different DPI won't help - it's useless for both development purposes and presenting to the client. Displaying the design on computer screen will never gove you the same look and feel as on mobile. You will not achieve the same results as on mobile. Some reasons for that are:
This will not let you judge if design is good or bad just by overviewing the graphics on the computer screen. Moreover, there is a problem when presenting these to the client, as the he's going to use different screen and the experience will be different again. From my experience, the best way to present designs to the client is to prepare a quick prototype to display on the screen of the device it is going to be used on (same contrasts, and sizes), or at least tell the client to view it on such device. The same for your internal testing of course.
Some of the most commonly made mistakes are too small clickable elements and fonts, and not considering different screen ratio between iPhone 5 and 4s and lower.
Some quick design tips to let you start with it:
You can find guidelines for iOS here: http://developer.apple.com/library/ios/#documentation/UserExperience/Conceptual/MobileHIG/UEBestPractices/UEBestPractices.html. Unfortunately, this topic is too broad to describe it here, you need to study it, understand it and then apply it. Misunderstanding these (e.g. the 44px principle) may lead to design errors as well.