Eleven algorithms for the passive-microwave measurement of sea-ice were implemented and inter-compared. Daily, monthly and annual Arctic sea-ice concentration, area and extent were calculated by the algorithms using daily microwave brightness temperatures from SMMR, SSM/I, and SSMIS for the period 1979-2012. The differences between the 11 sea-ice concentration estimates-structural uncertainties-were quantified and analyzed spatially and seasonally. The algorithms differ in annual sea-ice area by 0.0-1.3 million km² and in extent by 0.0-0.6 million km². Linear trends for 34- and 21-year periods were calculated and compared for sea-ice concentration, area and extent. Low-frequency algorithms obtained annual Arctic sea-ice area decrease of 0.534-0.573 million km² per decade (0.439-0.491 million km² per decade for the extent) over the period 1979 to 2012, and a decrease of 0.866-0.975 million km² per decade (0.767-0.812 million km² per decade for the extent) for the 1992-2012 period. High-frequency algorithms obtained a decrease of 0.766-0.978 million km² per decade in the area and 0.758-0.814 million km² per decade in the extent over the period 1992-2012. Results for all the algorithms have close agreement on the strength of the negative trend in Arctic sea-ice area and extent, but are individually biased from the mean. The algorithms' ensemble mean and standard deviation in sea-ice concentration, describing part of the uncertainty, are presented to provide users with more insight into the uncertainties and potential biases of sea-ice concentration data.