SANTA BARBARA, CA—If the proof of the pudding is in the eating, as the old expression has it, then the proof of a property sector's strength is in its metrics. On that score, investors' confidence in multifamily continues to be justified, with both Yardi and MPF Research reporting year-over-year gains in apartment rents and occupancy.

Yardi Matrix data released Wednesday showed that rents rose in 2015 by an average of 6.4% across the 111 metro areas it tracks. That national average represents the second highest increase over the past decade and 190 basis points more than the 4.5% recorded in 2014, according to the December 2015 edition of Matrix Monthly. It's the case even as December rents were basically flat, as they have been for the past four months.

Helping push the average growth rate up across the board were year-over-year increases in the top five metros, all on the West Coast. Portland, OR led the way at 14.8%, followed by Sacramento (10.7%), Seattle (10.5%), San Francisco (11%) and Los Angeles (9%).

Rents averaged $1,165 nationwide in December, $1 less than the all-time peak reached in September and October, consistent with normal seasonal flattening, according to Yardi. Similarly, Carrollton, TX-based MPF reported a 0.1% decline in year-end rents, with an average of $1,244 per month across the 100 largest metros.

The rental market intelligence division of RealPage, MPF said Monday that rents for new residents at apartment communities rose by an average of 4.8% in '15. It marks six consecutive years of Y-O-Y increases at or above the norm of 2.7%.

"Continuing strong rent growth reflects the fact that product availability remains unusually low," says Greg Willett, chief economist at RealPage As a case in point, fourth-quarter occupancy ticked upward by three bps from a year ago to 95.8%.

Willett notes that delays in deliveries, due in large measure to labor shortages in select building trades, are having a significant impact on the apartment market's overall performance. "Pushing back completion dates two or three months is translating to a longer pre-leasing period for new properties coming on stream," he says. "Those new projects are finally opening with better-than-expected occupancy, and that occupancy premium helps boost rent growth."

MPF says a total of 232,168 units were completed across the country's 100 largest markets last year, falling short of the originally projected 300,000 units by more than 20%. As a result, new supply in '15 slid 8% from 2014's tally of 252,348 units delivered, rather than increasing by 20% as was expected.

Currently there are 443,240 units under construction, including 311,511 units slated for delivery this year. Accordingly, next year's new supply could top 2015's total by as much as 34%.

"That many additions would likely lower occupancy slightly and slow rent growth moderately," says Willett. "However, if 20% to 30% of 2016's scheduled deliveries are delayed until 2017—in keeping with what the sector experienced last year—"it wouldn't be especially surprising to see another year of occupancy and rent growth like 2015."

All things being equal, Yardi expects rent growth to cool somewhat in the future, while remaining "above the historical average. Nationwide, demand will remain robust as the large millennial generation reaches renter age and many form households, while empty-nest baby boomers increasingly trade down from oversized houses."