Best judgment assessment: officer may determine income after considering materials and offering an opportunity to be heard. Best judgment assessment authorises the assessing officer to determine total income or loss and the sum payable when an assessee fails to file required returns or comply with statutory notices; the officer must consider relevant materials, give the assessee an opportunity of being heard by serving a show cause notice specifying date and time, but that opportunity need not be given where a prior statutory notice has already been issued.