Massachusetts retailers are expecting $15.8 billion in sales through November and December, marking what would be the sixth consecutive year of holiday sale increases.

Based on a survey of its 3,500 members, the Retailers Association of Massachusetts is predicting a 6.5 percent growth in 2015 holiday period sales over last year, well above the National Retail Federation's projected uptick of 3.7 percent in sales nationwide.

In 2014, Massachusetts retail sales in November and December rose by 3.4 percent over the previous year, with national sales increasing by 4.1 percent.

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Retailers association president Jon Hurst said that members of his statewide trade group have reported higher inventory levels and a slight rise in seasonal store hiring, and are preparing for the holiday sales season with "increased optimism."

"Consumer confidence is at the highest local level we have seen since the 2007 holiday sales season," Hurst said in a statement. "Economic growth, lower unemployment, and the lowest gas prices we have seen in five years have a lot to do with that boost in consumer confidence, and we have seen that reflected in retail sales growth in recent months. Low consumer debt levels, and improving home and 401k values are helpful to boost confidence levels as well. And given a very competitive marketplace, consumers will once again enjoy an environment of extensive promotions and value from a multitude of shopping options."