By Angela Chen 

Shares of big data firm Hortonworks Inc. and app-diagnostic tool New Relic Inc. surged in their public trading debuts Friday, likely a relief for the bankers and venture-capital firms that previously invested in the startups at hefty valuations.

Hortonworks, whose initial public offering priced at $16 a share, opened up 50% at $24 a share. New Relic, meanwhile, priced at $23 a share and opened 31% higher at $30. In recent trading, Hortonworks was up 44% and New Relic continued climbing, rising 45%.

Both companies proposed earlier this month to sell shares to the public at sharp discounts to their valuations, fanning worries about overly generous valuations across the sector and a larger tech bubble.

But Hortonworks' offering priced above its estimated range of $12 to $14 a share, and Friday's share pop puts its valuation at about $940 million. New Relic, for its part, raised its estimated price range ahead of the deal and its valuation is now about $1.5 billion.

Palo Alto-based Hortonworks develops Hadoop, a framework that helps process large data sets and boasts Yahoo Inc. as its top outside investor, while New Relic's software helps website or app owners track the performance of their services.

Other companies in the tech space such as Uber Technologies Inc., software provider Dropbox Inc. and data-mining startup Palantir Inc. have attracted multibillion price tags in a relatively short period. Ride-hailing app company Uber, especially, has been valued at a near-record $41 billion.

But concerns about the sector have mounted when Hortonworks and New Relic joined the list of at least 30 other companies that have gone public in the U.S. this year with lower prices than they were worth in private stock sales or option grants in the prior 90 days, according to Valuation Advisors LLC, which conducts valuations for private companies. That compares with 10 such companies last year.

As a market correction has battered valuations for public companies in similar sectors as Hortonworks and New Relic, some IPO prices are now being set in line with those lower valuations. Other companies, such as Box. Inc., have decided to delay IPOs as market conditions deteriorate.

The $25 billion IPO of Chinese giant Alibaba Group Holding appeared to make 2014 a banner year for IPOs. The cumulative $38.9 billion raised is the most since the 2000 dot com boom. Yet when Alibaba is taken out of the picture, the remaining $13.9 billion is much closer to the average tech-IPO total of $11.5 billion since 1995.

Part of the concern surrounding Hortonworks and New Relic is their modest revenue in relation to their valuations.

For the nine months ended Sept. 30, Hortonworks recorded $33.4 million in revenue, which doubled last year's mark but wasn't enough to outweigh expenses, leading to a near doubling of its loss to $86.7 million.

Similarly, San Francisco-based New Relic saw a doubling in revenue this year to $63.2 million, but continued to operate at a loss.

Friday's pop for Hortonworks and New Relic followed a strong showing Thursday from Lending Club Corp., which connects borrowers with lenders online for a fee. It shares surged 56% from the IPO price, a strong debut for the first publicly traded peer-to-peer lending company.

Write to Angela Chen at angela.chen@wsj.com

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