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    Thinking Dictionary

    Senses vs instruments

    What it is

    Our senses, such as sight, hearing, touch, and smell, are fast and useful. But they are limited, and they are easily fooled. Instruments, such as thermometers, scales, and satellites, can measure things we can't sense at all. They also tend to give more consistent readings. But instruments can be set up wrong, read wrong, or simply break. The best approach is to let each one check the other.

    Why it matters

    "Seeing is believing," the saying goes. But what we notice depends on where our attention is and what we expect.

    In a famous experiment, people watched a short video of players passing a basketball. Their task was to count the passes. Partway through, a person in a gorilla suit walked through the scene. Many of the viewers never noticed the gorilla at all. How many noticed depended on how hard the counting task was.¹ This is called inattentional blindness: missing something in plain sight because your attention is busy elsewhere.

    Instruments don't get distracted like that. But they can be confidently wrong. In 2011, a physics experiment called OPERA reported tiny particles, called neutrinos, travelling faster than light. The cause turned out to be faults in the equipment.² Further checking found them, and the corrected result agreed with the speed of light.³ Closer to home, wrist-worn fitness trackers can be off by 20% or more when they estimate calories burned.⁴

    A real case: a lake seen from space

    In late 2009, a large bloom of cyanobacteria appeared on Lake Atitlán in Guatemala. Cyanobacteria are tiny organisms, sometimes called blue-green algae, that can turn water green and make it harmful. Nothing like it had been seen there before. At its peak, it covered about 40% of the lake's surface.⁵˒⁶

    From a boat, you can only see the water around you. Satellite images showed the whole bloom at once, so officials and the public could see how big it was.⁵˒⁶ Africa Flores, a scientist working with NASA, and her colleagues later went further. They used satellite data to estimate the amount of chlorophyll, the green pigment in algae, across the lake. They checked those estimates against water samples taken by hand from the lake.⁶ Checking remote readings against direct samples on the ground is called ground-truthing. Here, the satellites and the samples checked each other.

    In everyday life

    • "It feels warmer today." How warm a room feels depends on whether you just came in from the cold. A thermometer doesn't care where you've been.
    • Your fitness tracker. It can count steps and measure heart rate tirelessly. But its calorie estimates can be well off, even though they look precise.⁴
    • The smell of gas or a strange taste in the water. Instruments don't always win. In Flint, Michigan, people noticed discoloured water before the official alarm.⁷ Your senses can be an early warning, even if they can't tell you how big the problem is.

    The common trap

    There are two traps, and they point in opposite directions.

    The first is trusting your senses too much. Attention and expectation filter what you notice, so "I'd have seen it" is weaker evidence than it feels.¹

    The second is trusting the machine too much. "The device said so" is only as good as the device and the way it was set up and checked.³

    Some ways out:

    • When an instrument gives a surprising result, ask how it was checked.
    • When your senses tell you something, ask whether an instrument could confirm it.
    • When the two disagree, treat that as a clue worth following up.

    Try it

    You need three bowls of water: one cold, one lukewarm, and one warm. Keep the warm one comfortably warm, never hot. Put one hand in the cold bowl and the other in the warm bowl for 30 seconds. Then put both hands into the lukewarm bowl.

    One hand will say the water feels warm. The other will say it feels cold. It is the same water.

    Now measure the lukewarm water with a thermometer. Which do you trust more: your hands or the thermometer? And how could you check the thermometer itself? One way is to put it in a glass of ice water, which should read about 0 °C.

    Related concepts

    Accuracy vs precision · Multiple lines of evidence · Confirmation bias · Iteration · Random vs systematic error

    Sources

    1. Simons, D. J., & Chabris, C. F. (1999). Gorillas in our midst: Sustained inattentional blindness for dynamic events. Perception, 28(9), 1059–1074. https://doi.org/10.1068/p281059

    2. Reich, E. S. (2012). Flaws found in faster-than-light neutrino measurement. Nature (news). https://doi.org/10.1038/nature.2012.10099

    3. Adam, T., et al. (OPERA Collaboration). (2012). Measurement of the neutrino velocity with the OPERA detector in the CNGS beam. Journal of High Energy Physics, 2012(10), 093. https://doi.org/10.1007/JHEP10(2012)093

    4. Shcherbina, A., et al. (2017). Accuracy in wrist-worn, sensor-based measurements of heart rate and energy expenditure in a diverse cohort. Journal of Personalized Medicine, 7(2), 3. https://doi.org/10.3390/jpm7020003

    5. NASA Earth Observatory. (2009). Harmful bloom in Lake Atitlán, Guatemala. https://science.nasa.gov/earth/earth-observatory/harmful-bloom-in-lake-atitlan-guatemala-41385/

    6. Flores-Anderson, A. I., Griffin, R., Dix, M., Romero-Oliva, C. S., Ochaeta, G., Skinner-Alvarado, J., Ramirez Moran, M. V., Hernandez, B., et al. (2020). Hyperspectral satellite remote sensing of water quality in Lake Atitlán, Guatemala. Frontiers in Environmental Science, 8, 7. https://doi.org/10.3389/fenvs.2020.00007

    7. Pieper, K. J., Tang, M., & Edwards, M. A. (2017). Flint water crisis caused by interrupted corrosion control: Investigating "ground zero" home. Environmental Science & Technology, 51(4), 2007–2014. https://doi.org/10.1021/acs.est.6b04034