Amazon Prime, the e-commerce giant’s subscription membership service, is now being used by 38% of American households, according to Investors Business Daily.

As part of the company’s subscription service, Amazon Prime, users can access streamed movies, TV shows and Amazon’s original productions, in addition to free two-day shipping on orders.

Investment bank Cowen, which surveyed 2,500 U.S. shoppers, said in a research report Wednesday that Amazon Prime had 41 million members in December, a 32% increase from the same month a year earlier. Cowan partly attributed the growth to a Black Friday-like sales holiday Amazon held for Prime members over the summer that attracted new subscribers.

Some estimates peg Amazon’s worldwide Prime membership at 80 million.

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Amazon Prime members are also reportedly higher earners than the average Amazon, Walmart , and Target customers. In December, Cowen released a report showing that Prime Members have an average household income of $69,300, well above the income of Amazon shoppers in general. It is also 25% higher than Walmart shoppers’ average income and 4% above Target’s. The point is that Prime is winning the war against other big retailers to attract higher income shoppers.

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Clearly, Prime continues to grow, which means it’s harder for Walmart, Target, and others to compete with the Seattle-based company. Both Walmart and Target reported modest e-commerce growth rates in the third quarter, as Amazon’s sales grew 23%.