Experimental Error
Experimental error is the difference between a measured value and the true value, arising from random variation or systematic flaws in method or instrument.
Definition
Experimental error is the small difference between what you measured and the real value, present in every experiment because tools and people are not perfect. Random error comes from unpredictable small variations (reaction time, reading a scale) and scatters results above and below the true value, so it is reduced by averaging more trials and quantified by standard deviation, shrinking as $1/\sqrt{n}$. Systematic error comes from a flaw that pushes every measurement the same direction (a miscalibrated balance, a ruler starting at $1$), cannot be fixed by averaging, and must be identified through controls, calibration standards, or independent methods. Percent error compares a result to an accepted value, and uncertainty analysis propagates instrument resolution through the calculation.
Example
Three students time the same cart and get $1.8\ \text{s}$, $1.9\ \text{s}$, and $1.7\ \text{s}$; nobody made a mistake, their reaction times just differed. A balance that reads $0.5\ \text{g}$ with nothing on it adds $0.5\ \text{g}$ to every mass (systematic), while stopwatch times varying by $\pm 0.2\ \text{s}$ from reaction time are random. A group measures water density as $1.03\ \text{g/cm}^3$ against the accepted $1.00\ \text{g/cm}^3$, a percent error of $|1.03 - 1.00|/1.00 \times 100\% = 3\%$; since their five trials range only $0.01\ \text{g/cm}^3$, the error is not scatter, and the balance is found to read $0.3\ \text{g}$ high with an empty pan.
Key Insight
Error in science does not mean "mistake"; it means the normal wobble in measurements, and good scientists measure how big it is instead of pretending it is zero. Human slips like misreading or miscopying are not experimental error in a lab report, so write about the real sources: instrument limits, reaction time, temperature drift, and how you tried to reduce them. Random error limits precision and systematic error limits accuracy, so diagnose which kind you have before deciding whether the fix is more trials or a better calibration.