UX and UI are used interchangeably in sales material and mean different things in practice. UX is research and structure: who the user is, what they are trying to do, and what the flow should be. UI is the visual and interaction layer built on top. A team strong at one is often mediocre at the other, and knowing which you are short of saves a lot of money.
UI UX design services are three purchases: research, structure and surface
Research answers who the users are and what they are trying to achieve, and is the part most often skipped because it produces documents rather than screens. Structure decides information architecture and flows, which is where most usability is won or lost. Surface is the visual and interaction design people usually mean by design. A quote that covers only the third will produce something attractive that still confuses people, and the fault will be attributed to the wrong thing.
Judge the best UI UX design agency on evidence, not on portfolio beauty
Portfolios select for what photographs well. What predicts a good outcome is whether the agency tests with real users and changes its work when the testing says so. Ask for an example where usability testing changed a design, and what it changed. The W3C's guidance on involving users in web projects is a reasonable reference point for what genuine user involvement looks like, and an agency that treats users as reviewers of finished work rather than participants is not doing it.
In UI UX services, accessibility is design work, not a development task
Most accessibility failures originate in design: insufficient contrast, meaning carried by colour alone, custom controls with no keyboard model, focus order that follows the visual layout rather than the logical one. Developers can implement an accessible design and cannot rescue an inaccessible one cheaply. Name a conformance level from the W3C's Web Content Accessibility Guidelines in the design brief and require the agency to show how its designs meet it.
UI UX design and development services: design that ignores implementation gets rebuilt
A design produced without reference to what is being built with it tends to specify things that are expensive or impossible, and then gets quietly redrawn by developers under time pressure, which is the worst of both. Insist that design and engineering talk before the design is finished, ask for a component based deliverable rather than a set of unique screens, and require states, empty, loading, error, long content, to be designed rather than invented later.
Questions people ask about ux services
What is the difference between UX and UI in UX/UI services?
UX is research and structure: who the user is, what they are doing and what the flow should be. UI is the visual and interaction layer on top. Agencies are usually strong at one, so establish which you are short of before shortlisting.
How do we judge a UX designer agency?
Ask for an example where usability testing changed a design and what it changed. Portfolios select for what photographs well; willingness to be corrected by real users predicts the outcome much better.
Whose job is accessibility?
Mostly design's. Contrast, meaning carried by colour, custom controls without a keyboard model and illogical focus order all originate in design. Name a WCAG conformance level in the design brief rather than leaving it to development.
Should UI/UX development services put design and engineering together?
Yes, before the design is finished. A design made without reference to what is being built tends to specify things that are expensive or impossible, and then gets quietly redrawn by developers under time pressure. Ask for a component based deliverable and designed states rather than a set of unique screens.