Ramana - there doesn't really exist any current system that equates author order to a credit system. It would be nice to think that each author, in turn, is given status according to how much they have contributed to the submitted manuscript. It's not always the case - such as when an author may have contributed nothing - but has agreed to lend their established name to the study. Alternatively, the first named author may be an 'unscrupulous' supervisor who has 'hijacked' the study and given themselves more credit that they are due. Either way, without knowing these personal 'ins and outs' - there is no way of knowing what contribution is 'equal'.
Dear sir perfectly said: many a times we just add names to the paper in respect of our peers: as I did: but then the first author should never the less take major chunk of the papers merit, almost the same with the last author who mostly will be the supervisor or the senior most peer.
the question then remains of the remaining contributors: I believes we must include authors in a paper who either significantly contribute in work or writing and they will almost get the same credit but surely not more than the first author:
Agreed - the first author should have performed the 'lion's share' of the work; so formulated the original research idea, supervised/conducted most of the study design and having formulated and formatted most of the manuscript for submission. Middle authors will have assisted in study design I.e. data collection and analysis - and helped to refine the original manuscript draft - in a perfect world that is!!