SLA vs SLO vs SLI: How Reliability Rebates Turn Service Metrics Into Outcomes

Key takeaways

• An SLI is what you measure, an SLO is the target you set for it, and an SLA is the contract that says what happens if the target is missed. 

• Most IT contracts get the SLA part wrong: service credits are capped, hard to claim and measured too coarsely to change behaviour. 

• A Reliability Rebate uses the same SLIs, measured continuously, and applies a defined credit to the invoice automatically when a service level is missed. 

SLAs, SLOs and SLIs come from site reliability engineering, where they help engineering teams decide how reliable a system needs to be. Most explanations stop there. But for anyone buying IT services, the same three ideas decide something more practical: what you’re actually guaranteed, how it’s measured, and what happens to your bill when it isn’t delivered. 

This guide explains all three with IT service examples, shows how to measure them properly, and walks through how a Reliability Rebate turns them into a commercial consequence. 

SLA vs SLO vs SLI: the definitions

An SLI (service level indicator) is a measurement of how a service is performing. An SLO (service level objective) is the target value for that measurement. An SLA (service level agreement) is the contract that commits to the target and sets out the consequence of missing it. Put simply: the SLI is the number, the SLO is the goal, and the SLA is the promise. 

	SLI	SLO	SLA
Stands for	Service level indicator	Service level objective	Service level agreement
What it is	A measurement	A target for that measurement	A contractual commitment
Example	Share of P1 tickets responded to within 15 minutes	95% of P1 tickets responded to within 15 minutes, monthly	If the 95% target is missed, a defined credit applies
Who uses it	Engineers and service managers	Service owners	Commercial and legal teams, and buyers
If it’s missed	Nothing by itself; it’s just data	A signal to act internally	A contractual consequence

 

Common SLIs for IT services

In IT managed services, the most useful indicators are the ones a business user would notice: 

• Availability: the share of time a service is usable. 

• Response time by priority: how quickly tickets are picked up, measured separately for P1, P2 and so on. 

• Mean time to resolution (MTTR): the average time from a ticket being raised to it being fixed, ideally reported by category. 

• First contact resolution: the share of tickets fixed at the first attempt. 

• Auto-resolution rate: the share of tickets resolved with no human involvement. In Allied’s service desk operations, 84%+ (Allied’s stated figure). 

• User satisfaction: how people rate the help they received. 

Several of these are explained in more depth in our guide to service desk automation. 

How to measure an SLI properly

Most disputes about service levels are really disputes about measurement. Four details matter: 

1. The formula. An SLI is usually a ratio: good events divided by valid events. For P1 response: tickets responded to within 15 minutes ÷ all P1 tickets in the period. 
2. The measurement window. A calendar month, or a rolling 30 days. Rolling windows show trends faster; calendar months line up with invoices. 
3. Exclusions. Planned maintenance, issues caused by the client, or events outside the provider’s control. Exclusions should be listed precisely, not left open to interpretation. 
4. The data source. Measured from the ticketing and monitoring systems, automatically and continuously, not compiled by hand at month end. 

What is an error budget?

An error budget is the amount of unreliability an SLO allows. It’s calculated as 100% minus the SLO. If an availability SLO is 99.9% over a 30-day month, the error budget is 0.1% of 43,200 minutes: 43.2 minutes of allowable downtime. 

Error budgets are useful for buyers too. They turn an abstract percentage into something concrete, and they make it obvious when a service is burning through its allowance early in the month. 

SLA compliance: why service credits rarely work

Most IT contracts attach service credits to their SLAs: a refund if targets are missed. In practice, they rarely change anything, for five reasons: 

• They’re capped. Total credits are usually limited to a small share of the monthly fee, so missing targets badly costs the provider little more than missing them slightly. 

• You have to claim them. The burden is on the client to spot the miss, prove it and raise a claim, often within a short window. 

• They’re measured too coarsely. A single monthly average can hide a terrible week inside an acceptable month. 

• Exclusions are broad. Vague exclusion clauses let many misses fall outside the calculation. 

• They measure the wrong things. Response time is easy to hit by acknowledging a ticket; resolution is what users care about. 

How a Reliability Rebate works

Allied’s Reliability Rebate is the credit-back mechanism in our Flexible Resource Architecture (FRA™), and it’s designed to fix each of those problems: 

• Same SLIs, measured continuously. Thresholds are tracked from live system data throughout the period, not reviewed at the end. 

• Applied automatically. When a service level is missed, the rebate is applied to that period’s invoice without a claim or dispute. 

• Calculated consistently. A defined formula links the size of the shortfall to the size of the rebate, agreed up front. 

• Focused on outcomes. Resolution, first contact resolution and auto-resolution sit alongside response times, so the measures reflect what users experience. 

 

 Worked example (illustrative figures only)

The numbers below are illustrative, to show how the mechanism works. They are not Allied’s commercial terms. Actual thresholds and rebate rates are agreed per contract. 

Suppose a contract sets this SLO: 95% of P1 tickets responded to within 15 minutes, measured per calendar month. In one month there are 200 P1 tickets, and 182 are responded to within 15 minutes. 

1. SLI = 182 ÷ 200 = 91%. 
2. Shortfall against the 95% SLO = 4 percentage points. 

The contract defines rebate tiers in advance: 

Performance against SLO	Shortfall	Rebate on that service line (illustrative)
At or above 95%	0 points	None
92% to below 95%	More than 0, up to 3 points	Tier 1 rebate
88% to below 92%	More than 3, up to 7 points	Tier 2 rebate
Below 88%	More than 7 points	Tier 3 rebate

  

At 91%, the month falls into Tier 2, so the Tier 2 rebate is applied to that month’s invoice automatically. Nobody has to notice, calculate or claim it. 

Beyond SLAs: experience level agreements

Some organisations add XLAs (experience level agreements) that measure how users feel about the service, not just whether technical targets were met. XLAs work best alongside clear SLIs, not instead of them: satisfaction tells you something is wrong, while the SLIs tell you what. 

Questions to ask about any IT services SLA 

1. Which SLIs are measured, and are they the ones our users would notice? 
2. How exactly is each SLI calculated, over what window, and with what exclusions? 
3. Is measurement continuous and automatic, or compiled manually? 
4. What happens when a target is missed, and do we have to claim it? 
5. Are credits capped, and at what level? 
6. Can we see live performance against each SLO during the month? 

Conclusion

SLIs tell you what’s happening, SLOs tell you what should be happening, and SLAs tell you what happens when the two don’t match. Most IT contracts are strong on the first two and weak on the third. A Reliability Rebate closes that gap by measuring continuously and applying the consequence automatically. It’s the commercial side of the outcome-based model explained in our guide to managed IT services pricing. 

Want to see how your current SLAs would look under a Reliability Rebate? Talk to our team and we’ll walk through it with you.