In the name of convenience, Amazon and Walmart are pushing people to shop by just talking to a digital assistant. Shopping by voice means giving orders to the Alexa assistant on Amazon's Echo speaker and other devices, even if your hands are tied up with dinner or dirty diapers. And next month, Walmart will start offering voice shopping , too, with the Google Assistant on the rival Home speaker. Voice shopping is still new. But once you start using it, look out - you might never know if it's offering you the best deal. Because these devices can't say much without tiring your ears, voice shopping precludes some of the savvy shopping practices you may have relied on to find the best bargains - in particular, researching products and comparing prices. You'd be leaving much of the buying decision to Amazon, Walmart or other retailers.

Amazon has had more than a year's head start, and dominates voice shopping. Google introduced shopping to Home in February, letting people order essentials from more than 40 retailers like Target and Costco under its Google Express program. Its partnership with Walmart means hundreds of thousands of items will be available to customers in late September.

With websites and apps, many customers place items in the cart, but change their minds before completing the order, said Lauren Beitelspacher, a marketing professor at Babson College in Massachusetts. Voice shopping eliminates those intervening steps. And with Amazon so far ahead, voice shopping with Alexa is another way of getting you hooked on Amazon. Although Amazon allows some third-party ordering through Alexa, including pizza from Domino's and hotels through Kayak, general shopping is limited to Amazon's own store. If Alexa orders diapers for you just as you run out, for instance, Amazon locks in the order before you have a chance to visit Walmart.

"You can't get away from Amazon," Beitelspacher said. "I don't know if gimmick is the right word, but (voice shopping) is part of a strategy to be omnipresent in consumers' lives."

Ask Alexa to buy something, and it presents you with something you've bought before or an educated guess based on some undisclosed mix of price, satisfaction rating and shipping time. Amazon won't provide more details. You can get a product's average customer-satisfaction rating, but not specific reviews, even on screen-equipped Echo Show devices. Brian Elliott, general manager of Google Express, says that with most affiliated retailers, personalization occurs as the assistant learns shoppers' preferences, but the integration with Walmart will happen more quickly. In some ways, shopping by voice assistant is a throwback to the days when you were largely limited to what sales representatives recommended at a physical store.

Amazon's website gives you a lot of information about most products, from color options and sizes to the specific reasons other customers hated a product you're considering. You're able to compare similar items and choose something cheaper if you're willing to sacrifice some features or take a chance on an unknown manufacturer. And, of course, you can also compare Amazon's prices with those of other online merchants. But with Amazon's voice shopping, it's back to what the company's representative recommends. Voice shopping requires membership in Amazon's $99-a-year Prime loyalty program, and it works with most of the tens of millions of items eligible for free shipping. But someone browsing on the web might find deals in non-Prime items; Alexa won't let you buy them. In addition, Alexa's interactions with shoppers are constrained by the fact that listening and speaking can be a lot slower than reading and clicking. And while Amazon's website won't necessarily list the cheapest option first either, the alternatives are easier to view on a screen.