SAN FRANCISCO—Lawyers for Twitter Inc. pushed back against an attempt to certify a class of female engineers in a gender discrimination case Wednesday.

Twitter's lead lawyer, Lynne Hermle of Orrick, Herrington & Sutcliffe, told San Francisco Superior Court Judge Mary Wiss that the plaintiff couldn't show that the company's promotion practices caused a disproportionate number of women to be passed over for promotions.

The lawsuit, brought on behalf of former Twitter engineer Tina Huang who joined the company in 2009 as one of its first female software engineers, seeks to certify a class of 135 current and former women employees who held similar positions. Huang and her lawyers at San Francisco's Lohr Ripamonti & Segarich claim that Twitter managers used subjective criteria for promotion into senior positions, which led to discrimination against female candidates.

Huang's lead lawyer, Jason Lohr, said at Wednesday's hearing that Twitter was attempting to treat the class certification stage of the case as summary judgment by raising evidentiary issues.

“It's a procedural motion and the standards are very different,” Lohr said.

Lohr said that, at this stage, the appropriate question is whether Huang's claims were typical of the class, whether the case presents common questions of fact and law and whether a single case is superior to 135 single cases. He also said that all plaintiffs can claim that Twitter's promotional practices led to a disparate number of women remaining in the lower rungs of the company's eight-tier hierarchy for software engineers.

In particular, he pointed to the company's practice having supervisors act as “gatekeepers” who suggest which employees are eligible for promotion, and its subjective criteria requiring candidates to be judged by the impact their work had on the company and how that was accomplished.

Hermle, however, pointed out that Huang's statistical expert hadn't reviewed Twitter's policies or any deposition testimony, but had relied purely on information provided to him by Lohr. That reliance, Hermle contended, meant that the statistical evidence that the plaintiff put forward was inadmissible.

“There's nothing scientific about it,” Hermle said. “There's nothing that would have allowed a reasonable expert to rely on it.”

Wiss, for her part, seemed troubled that some of the potential class members had sat on internal Twitter committees who considered the promotion prospects of other class members.

“Why doesn't that create a conflict?” Wiss asked Lohr.

Lohr replied that the peer-review process at Twitter was typical of the industry and that, since all the prospective class members were female software engineers, all faced statistically dimmer prospects for promotion than their male colleagues.

Wiss, who did little to tip her hand one way or the other at Wednesday's hearing, also asked each side what she should do about the fact that 73 of the proposed class members either had agreed to arbitrate any claims against the company or had entered agreements upon leaving Twitter to waive future claims. Lohr said that those were issues that could be handled after a class is certified, class members are notified, and the parties see who opts out of the suit.

But Hermle said that fact that Huang has neither an arbitration agreement nor a waiver of claims leaves her unsuitable to represent those who do. She said that the fact the plaintiffs had not put forward another class representative who could represent plaintiffs in those positions after three-and-a-half years of litigation was telling.

“If the plaintiff wanted to have someone who is adequate and typical I would have expected that person to surface long before today,” Hermle said.