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How to justify your design decisions with psychology

By understanding the human mind, we can advocate for user-centred design. Psychological studies give us empirical evidence on how people behave. These studies can help create justified designs.

By understanding the human mind, we can advocate for user-centred design. Psychological studies give us empirical evidence on how people behave. These studies can help create justified designs.

I’ll discuss 5 principles from psychological studies that support good experiences. I’ll give examples from the SSE brand who are part of the OVO family.

Principle 1: Hick’s Law

The time it takes to make a decision increases with the number and complexity of choices available.

Avoid making interfaces complex and busy. Reduce people’s cognitive load (processing power) when using your product.

When we don't apply Hick's Law, we get overloaded with information. Have you ever struggled to make a decision about a purchase because there were too many options? That's information overload.

Example of Hick’s Law

SSE energy quote journey question regarding what electricity meter a customer has.

SSE uses a question and answer format when people are getting an energy quote. This format cuts down the appearance of a long-form. Progressive disclosure is our friend when we want to apply Hick’s law. Personal details, contact details, and property details are all separated out into different sections of the energy quote journey.

Principle 2: Peak-End Rule

People judge an experience based mainly on how they felt at its peak and its end, rather than on the total sum or average of every moment of the experience. People record ‘snapshots’ of experiences.

People make a judgement of products based on their most important actions. We need to learn what these actions are and create the best experiences for them. Following this rule facilitates a good user experience.

Think about a time you had a bad experience at a restaurant. It's likely that your experience wasn't awful from start to finish. What you remember about it is the 'peak'; the bit that went wrong.

Example of Peak-end Rule

SSE Smart Meter installation confirmation page that has a tick icon, positive heading and content in general.

SSE uses a short confirmation page at the end of the Smart Meter installation booking journey. Many people have low engagement with their energy providers. Booking a smart meter is one of the few times people will interact with their provider. SSE needs to make sure people are reassured about their booking. The positive headings and icon reinforce the process as quick and simple. Details about the installation provide clarity to customers.

Principle 3: Jakob’s Law

Users spend most of their time on other sites, and they prefer your site to work the same way as all the other sites they already know.

Design features based on patterns people understand. Learn your customer’s mental models.

Have you used a webpage and you just couldn't work out how to do something? It's likely that was because it was designed in a way that's inconsistent with how other web pages are designed and required you to work out a new interaction.

Example of Jakob’s Law

SSE recently added tariff details to the My SSE app. SSE did some competitor analysis to understand how this would work best for customers and learned that tariff management should sit within the main account space.

Principle 4: Doherty Threshold

Productivity improves when a computer and its users interact at a pace that is <400 ms. This ensures that neither has to wait on the other.

Facilitate a positive experience through feedback and quick load times. This is like Jakob Nielson’s heuristic: Visibility of system status.

We're all familiar with the frustrating experience of watching a webpage load very slowly. This creates a negative experience for people.

Example of Doherty Threshold

Example of loading modal on SSE experiences. There is an animation that assists the user.

This is a loading modal on SSE experiences when it takes longer than usual for something to process. The animation is engaging and reduces the perception of waiting.

Principle 5: Miller’s Law

The average person can keep only 7 (+2) items in their working memory.

Most people are only able to recall 7 items, and it’s difficult to keep information in working memory if it’s presented in an unstructured way. ‘Chunking’ can help people retain relevant information they want by breaking things down.

Have you ever read a product description full of paragraphs, and struggled to remember precisely what features are being described? That's because it's difficult to retain big, unstructured amounts of information.

Example of Miller’s Law

Onboarding experience that uses chunking. Content is chunked well using headings and bullet points.

SSE breaks down onboarding content into chunks. Here chunking is done using headings and bullet points. People can scan the information quicker, to see if it relates to their needs, and can retain the information better.

Thoughts and further reading

I wanted to share these principles after reading the Laws of UX.

Using psychological principles not only helps me design better experiences for customers but also helps me explain my design decisions to my colleagues. There are lots of ways you can do this, for example:

I encourage you to learn these principles and be a psychology advocate. Build awareness of the principles you learn. Doing this helps colleagues understand what contributes to good user-centred design.

Want to learn more?

Check Zarrin's post about Gestalt principles.

There is a great YouTube video running through some Laws discussed in Laws of UX.

Here are two more books I’ve read discussing psychology related to UX. They are ‘Bottlenecks’ by David C. Evans and ‘Design for How People Think’ by John Whalen.

I hope you found this post useful. Message me on LinkedIn if you want to chat.


Aminul Islam

UX Designer.

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