Google and AnswerLab conducted a research study to answer this question.
Mobile users are very goal-oriented. They expect to be able to get what they need, immediately, and on their own terms.
The study was held through 119 hour-long, in-person usability sessions with participants in the US. Participants were asked to perform key tasks across a diverse set of mobile sites. iOS and Android users were included, and users tested the sites on their own phones. For each site, participants were asked to voice their thoughts aloud as they completed conversion-focused tasks like making a purchase or booking a reservation.
The study uncovered 25 mobile site design principles, grouped into five categories.
Home page and site navigation
Keep calls to action front and center
Make secondary tasks available through menus or “below the fold” (the part of the webpage that can’t be seen without scrolling down).
Keep menus short and sweet
Mobile users don’t have the patience to scroll through a long list of options to find what they want. Reorganize your menu to use as few items as possible, without sacrificing usability.
Make it easy to get back to the home page
Users expect to go back to the homepage when they tap the logo in the top-left of a mobile page, and they become frustrated when it isn’t available or doesn’t work.
Don't let promotions steal the show
Large app install interstitials (e.g., full-page promotions that hide content and prompt users to install an app) annoy users and make it difficult to perform tasks. In addition to annoying users, sites that use interstitials may see a negative impact to their search rankings.
Make site search visible
Users looking for information usually turn to search, so the search field should be one of the first things they see on your pages. Don’t hide the search box in a menu.
Ensure site search results are relevant
Users don’t scan through multiple pages of results to find what they’re looking for. Make life easier on users by auto-completing queries, correcting misspellings, and suggesting related queries. Rather than reinventing the wheel, consider robust products like Google Custom Search.
Implement filters to narrow results
Study participants rely on filters to find what they’re looking for, and abandon sites that do not have effective filters. Place filters above search results, and help users by displaying how many results will be returned when a specific filter is applied.
Guide users to better site search results
For sites with diverse customer segments, ask a few questions before presenting the search box, and use the customer’s responses as search query filters to ensure that users get results from the most relevant segment.
Commerce and conversion
Let users explore before they commit
Study participants were frustrated by sites that require upfront registrations to view the site, especially when the brand was unfamiliar. Although customer information may be integral to your business, asking for it too early may result in fewer registrations.
Let users purchase as guests
Study participants viewed guest checkouts as “convenient”, “simple”, “easy”, and “quick”. Users are annoyed by sites that force them to register for an account when making a purchase, especially when the benefit of an account is unclear.
Use existing information to maximize convenience
Remember and pre-fill preferences for registered users. Offer familiar, third-party checkout services for new users.
Use click-to-call buttons for complex tasks
On devices with calling capabilities, click-to-call links enable users to make a phone call by simply tapping a link. On most mobile devices the user receives a confirmation before the number is dialed, or a menu is presented asking the user how the number should be handled.
Make it easy to finish on another device
Users frequently want to finish tasks on other devices. For instance, they might wish to view an item on a larger screen. Or they might get busy and need to finish later. Support these customer journeys by enabling users to share items on social networks, or by letting users email themselves links from directly within the site.
Streamline information entry
Automatically advance to the next field when a user presses Return. In general, the fewer taps the user must perform, the better.
Choose the simplest input
Provide visual calendar for date selection
Clearly label start and end dates. Users should not need to leave a site and check a calendar app just to schedule a date.
Minimize form errors with labeling and real-time validation
Label inputs properly and validate input in real-time.
Design efficient forms
Take advantage of autofill
so that users can easily complete forms with pre-populated data. Pre-fill
fields with information you already know. For example, when retrieving shipping
and billing addresses, try to use
or enable users to copy their shipping address to their billing address (or vice versa).
Usability and form factor
Optimize your entire site for mobile
Use a responsive layout that changes based on the size and capabilities of the user’s device. Study participants found sites with a mix of desktop and mobile-optimized pages even harder to use than desktop-only sites.
Don't make users pinch-to-zoom
Users are comfortable with scrolling sites vertically, but not horizontally. Avoid large, fixed-width elements. Use CSS media queries to apply different stylings for different screens. Don’t create content that only displays well at a particular viewport width. Sites that force users to horizontally scroll fail the Google Mobile-Friendly Test, which may negatively impact their search rankings.
Make product images expandable
Retail customers expect sites to let them view high resolution closeups of products. Study participants got frustrated when they weren’t able to see what they were buying.
Tell users which orientation works best
Study participants tended to stay in the same screen orientation until something prompted them to switch. Design for both landscape and portrait, or encourage users to switch to the optimal orientation. Make sure that your important calls-to-action can be completed even if the users ignore the suggestion to switch orientations.
Keep your user in a single browser window
Users may have trouble switching between windows and might not be able to find their way back to the site. Avoid calls-to-action that launch new windows. Identify any journeys that might cause a user to look outside your site and provide features to keep them on your site. For example, if you accept coupons, offer them directly on the site, rather than forcing users to search other sites for deals.
Avoid "full site" labeling
When study participants saw an option for a "full site" (i.e., desktop site) versus a "mobile site", they thought the mobile site lacked content and chose the "full" one instead, directing them to the desktop site.
Be clear why you need a user's location
Users should always understand why you’re asking for their location. Study participants trying to book a hotel in another city became confused when a travel site detected their location and offered hotels in their current city instead. Leave location fields blank by default, and let users choose to populate them through a clear call-to-action like “Find Near Me”.