A precise measure is only a treasure if it’s accurate too! With the presidential debates taking center stage, discussions of candidate’s claim accuracy is all the rage. Scientists care about ACCURACY too, as well as PRECISION – yes there’s a difference between those 2!
PRECISION is a measure of CONFIDENCE in a measurement 😎 whereas ACCURACY is a measure of how close the measurement is to the TRUE value 🤥 So a candidate could tell the same lie over and over and stick true to their telling, thus being precise, but inaccurate.
💭 Imagine you’re playing pin the tail on the donkey 👇
🔹situation 1 👉 you’re confident that, even w/a blindfold, you know where the donkey’s bottom is 😎& you have perfect aim, so you pin the tail over & over in the same spot 🎯
🔸situation 2 👉 you’re not sure where the donkey’s bottom is exactly, so you try out a spots around where you think it is 🤷♀️
Now take off your blindfold 🙈🐵
🔹 In situation 1, if all the tails are close together AND in the correct location (donkey’s bottom) you were both PRECISE (because the tails are close together) ✅ & ACCURATE (because the tails are in the right spot) ✅
🔹BUT if you take off the blindfold & find that the tails are close together but what you thought was the donkey’s *rear* was actually its *ear*, you were still PRECISE ✅ BUT you are NOT ACCURATE ❌ 👉 you had false confidence 😩
🔸 In situation 2, if your tails were a bit separated, but they were all on the donkey’s bottom, you were NOT very precise ❌ BUT you WERE ACCURATE ✅
🔸 If the tails are separated & not on the donkey’s bottom then you were neither precise ❌ nor accurate ❌
So precision & accuracy measure 2 different things 🔑
Thankfully, in science, we don’t have to worry about molecules lying to us. But since they can’t directly “talk” to us, it’s up to us to interpret what they’re “saying” through making measurements – like how heavy something is (weight) or how much space it takes up (volume). The things we’re measuring can’t “lie” to us – but instead “lies” can be introduced accidentally when we “record” & “re-tell” their stories.
Imprecision is like if the story changes a lot. So this is easier to notice because you just have to ask a bunch of times (make multiple measurements). But accuracy’s harder to determine, because it requires you to know what the true story is! 👉 a good sign is if multiple different measuring tools give you the same answer 👍
So why might the story be “off”? When we can blame the equipment we call it INSTRUMENT ERROR but if we have no one to blame but ourself it’s OPERATOR ERROR 😔 But don’t feel bad! Some error is inevitable, you might even say it’s standard! Both operator & instrument can cause RANDOM ERRORS & SYSTEMATIC ERRORS👇
RANDOM ERRORS are “unbiased” 👉 this is like having a shaky arm that leads to poor aim 👉 your tails rarely directly hit their target 🎯 ❌ BUT they’re not biased towards any one direction (e.g. you don’t have a tendency to pin left of target vs right) & they vary in how “off” they are ⏩ as a result these errors “cancel out” if you average enough of them, so RANDOM errors do NOT affect average ❌ just the variability around the average ✅
SYSTEMATIC ERRORS, on the other hand, are BIASED (error’s always in a certain direction) & they don’t “fix themselves” 👉 this is like always pinning to the left of the target (or the right, or above, or below (but not all of them 🤓))
Since 👆 SYSTEMATIC errors are DIRECTIONAL, the average measurement *will* reflect this error 👉 if you always err to the left, so will your average
If you know the amount & direction of systematic error, you might be able to correct for it after the fact 🤞 If I keep pinning the tail on the donkey’s ear, I can “cheat” after the fact & move the donkey so the tails are in the right spot 👍 BUT I need to know how to move it 😩
Let’s look at an example -> in lab, we’re more often dealing w/meniscus bottoms, not donkey bottoms… 👉 when u measure liquids, edges of the liquid get pulled up walls of the container leading to a smile we call a meniscus http://bit.ly/2AeB1B6 & it’s important to measure from BOTTOM of meniscus ⚠️ BUT where bottom looks like it is depends on angle you’re looking from 👀 To get a good eye-level, direct, glance, adopt a “titration stance” 🤓
Still, chances are, despite good form, each time you go to measure, you’re looking from a slightly different angle, so location will be slightly different 👉 but different in different, random, ways 👉 1 time glance is more right-angled another time it’s more left-angled, etc. 👉 This would be an example of RANDOM ERROR & I learned it has a cool name 👉 PARALLAX ERROR 🤓
You can also get SYSTEMATIC ERRORS when measuring liquids 👉 They can be “fixable” like if you measure volume & then realize you left stir bar in 👉 you can subtract out volume of the stir-bar from the volume of the stir-bar + liquid
In 👆 case you can 👀 stir bar (or get an unwelcome surprise when you’re pouring the liquid & hear a thunk…). But the important thing is you found it!👍 What’s even more dangerous is when you don’t know that a systematic error’s there 😬 if you keep measuring & getting similar results, u might feel pretty confident 😎 why doubt it?🤷♀️ But what if marking lines on your graduated cylinder are off, so it’s like you always forgot the stir bar in there? 😬
Some systematic errors are harder to correct for even if you know they exist 👉 like if you only learn that you’re supposed to be reading from BOTTOM of meniscus *after* you’ve already made measurements 😩 If you’d been reading from top or edge of meniscus, your recorded values will ALL be too high, but how much “too high” is hard to know 😩
This post is part of my weekly “Bri-fings from the Bench” for The International Union of Biochemistry and Molecular Biology. Be sure to follow the IUBMB if you’re interested in biochemistry! They’re a really great international organization for biochemistry.