SAN FRANCISCO, Aug 18 (Reuters) - U.S. retailers looking for a much-needed sales boost may find a glimmer of hope in a survey released on Tuesday 18 August showing consumers have not yet finished the majority of their back-to-school shopping.

The average American family has finished only 41.6 percent of their back-to-school shopping as of Aug. 11, according to a survey conducted by BIGresearch for the National Retail Federation, a leading retail trade group.

In addition, the survey also found that nearly one-third of families with children in kindergarten through 12th grade have not yet started shopping.

When consumers are shopping, sales or coupons have influenced nearly half of their purchases so far this season.

"The days of mom or dad letting children put whatever they want in the shopping cart are over," said BIGresearch's Pam Goodfellow in a statement. "Parents ... are making careful spending decisions based on price instead of giving kids free rein to pick out whatever they want."

The back-to-school shopping season began in July and is the industry's second-biggest selling period after Christmas. It is being closely watched as a gauge of consumer spending and a harbinger of the upcoming Christmas sales season.

The NRF has already forecast a difficult back-to-school shopping season, finding in an earlier survey that the average family with children in kindergarten through 12th grade expected to spend 7.7 percent less on new school gear this year than last year.

Back-to-school shopping is expected to happen later in the season this year as states shifted tax-free holiday shopping days into August from July, and the timing of the Labor Day holiday means many schools will hold their first day of class later in September than they did a year ago.

"For any seasonal event, shopping happens closer to the event. We're seeing that in back-to-school," Wal-Mart Stores Inc (WMT.N) Chief Financial Officer Tom Schoewe told reporters last week after the retailer reported its earnings.

He said Wal-Mart was in the middle of the back-to-school season, and he felt "pretty good" about its position.

According to the NRF survey, six out of ten families will head to a discount store to do their back-to-school shopping. Nearly 45 percent said they will go to a department store, while 31.8 percent will head to a clothing store.

The survey also found that Wal-Mart, Target Corp (TGT.N), J.C. Penney Co Inc (JCP.N), Staples Inc (SPLS.O), Kohl's Corp (KSS.N) and Gap Inc's (GPS.N) Old Navy ranked among the top back-to-school websites.

The survey of 8,543 consumers was conducted from Aug. 4 through 11, and has a margin of error of plus or minus 1 percent, the NRF said. (Reporting by Nicole Maestri; Editing by Tim Dobbyn)