Those comparisons may be most useful within the same industry, however, since it's more accurate to compare environmental practices of companies that carry out similar operations.

The definition of 'green' here is of course Newsweek's alone, since there's no universally accepted standard for such a ranking. Also, the ranking doesn't appear to take into account supply chains–that is many companies, including consumer electronic companies, don't actually produce the products they sell, but source them from suppliers. That omission may have allowed HP and other consumer electronics companies to have gotten a bit of a free pass, since they weren't necessarily judged on the environmental impact of the manufacture of all their products.

The other report deliberately takes a narrower focus. In "Greening Consumer Electronics: Moving Away from Bromine and Chlorine," two non-profit environmental organizations, ChemSec and Clean Production Action, highlight seven companies, including Apple, Sony Ericsson and five component suppliers, that have "gone beyond compliance" (partly in anticipation of regulations) by claiming to have removed from their products most or all uses of the two widely used chemicals, which are linked to health and environmental problems.

While the report, which is really more directed at industry, provides some useful information for consumers looking for products made with fewer harmful chemicals, it only focuses on the two chemicals, and so doesn't give a full picture of the environmental impact of the company’s products in other respects.

In studies that have considered a range of other factors, including recycling practices and management of greenhouse gases, the companies have fared inconsistently. The Greenpeace electronics guide lists Apple in ninth place and Sony in third. But Apple placed 133rd in the Newsweek listing. (Sony, as a non-American company, is not listed.)

That inconsistency only illustrates how difficult it is to hold any one ranking or scoring of greenness as definitive, whether for products, corporations, or retailers such as Walmart.

For more, check our roundup of other information on the environmental aspects of electronics products and consumer-friendly tools such as EPAT (Electronic Product Environmental Assessment Tool) which allows buyers to identify and evaluate greener electronic products. —Kristi Wiedemann.