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Spatial Exploratory Data Analysis (SEDA): Maps, charts and statistical relationships Overview • Spatial Exploratory Data Analysis – Introduction • EDA • Spatial Analysis • SEDA – Maps – Distributions – Relationships • Outline of 2nd computer workshop • Requirements for 1 page report • NOT a review of statistics!!! Introduction: Doing research • Research is a process. It is iterative. It is messy and full of uncertainty and false steps. • „Scientific process‟ (inductive method – from empirical data to theory): – what are you interested in knowing? – how can this be formalised (hypotheses) – how can this operationalised (testable model) – gather the most appropriate data – validate the model (data analysis - this may or may not include formal statistical testing) – „laws‟ (firm, generalisable conclusions) -> Theory – in reality you will often use both induction and deduction • Application of formal statistical techniques (excel, Minitab, SPSS, SAS, GEODA,..) are just a small part of this process Exploratory Data Analysis • “Exploratory data analysis is an attitude, NOT a bundle of techniques” (Tukey 1977). • “Let data speak for themselves” • “Get a feel for the data” • Basically inductive (from data to hypotheses, theory,…) • Characteristics: Concentration on graphical procedures Stem and Leaf Diagram Spatial analysis • In essence geographical problems are about human activities which vary in space • This does not mean that we should ignore things that do not vary spatially! • But we are „experts‟ in looking at a phenomena spatially • We need to find out if there is or is not any spatial pattern • The essence of testing a geographical hypothesis is to find out whether or not there is any plausible reason why the phenomena varies in space Spatial analysis • Quantitative analysis can tell us if patterns we see are (statistically) significant (not “right” or “wrong”, only degrees of uncertainty) • Is a trend in our sample „real‟ or is it just a chance occurrence? • Positive and negative relationships are interesting • No discernible spatial causes (i.e. not statistically significant patterns) are also interesting because this will guide you to inquire further • Does where something happens influence why and/or how it happens? Exploratory Spatial Data Analysis • Exploring spatial patterns through maps, histograms, boxplots, scatterplots… • Identify outliers • Find “hotspots” • Formulate hypotheses • Look for statistical relationships between variables • Search for spatial spillovers Describing distributions • Mean • Standard deviation • Variance • Skewness • Kurtosis • Median • Quartiles, Percentiles • Inter-quartile Range (IQR) • Maximum, minimum values Looking for outliers: Boxplot Number of observations (London Wards) Outliers Hinge (1.5 times IQR) 75th percentile Median IQR 25th percentile Hinge (1.5 times IQR) Variable name: Percent students Looking for outliers: Histogram Mapping Linked windows / brushing Distributions and mapping BOXMAP Percent White British Standard Deviation Map Distributions and mapping Percent Bangladeshi Percent White British Distributions and mapping Relationships: Choosing a statistic depends on the questions you ask • What is A like? • Is A similar to B? • Is A different from B? • How much is A better/worse/different than B? • Are A and B related? [correlation] • Does A affect B? • Does A cause B? [regression] Are A and B related? • For example, you might want to know if the level of illness in an area is associated with poverty • Is there a relationship between health and wealth? • Do increasing poverty levels lead to increasing ill-health? • Do the variables co-vary consistently across space? • Easiest approach is to graph one variable against each other and look for associations • Association is seen in the pattern of points • Simplest pattern to spot and analyse is a linear relationship (i.e. resembles a straight line), although relationships could be curvilinear Is there a relationship? How strong? Does one cause the other? Identifying relationships with scatterplots Strong positive Strong negative Random Negative No relationship? Positive Relationship between variables (y = dependent; x = independent) Long term unemployment share = Constant + 0.3162*no qualification share + Error term Correlation between unemployment rate and long- term share of unemployed (standardized data) Correlation Coefficient now, how to objectively measure the strength and direction of these relationships? What is correlation? 0.73 • Correlation statistics allow you to measure the strength and direction of a association between two variables • Correlation provides a single number (correlation coefficient) that summarises level of variation between points (It is a standardised measure of covariance) • If a relationship is found, variables are said to be correlated • Useful for description, but also inferential (significance) Types of correlation Data type Nominal Ordinal Interval/ Ratio Display 2-way table 2-way table Scatterplot Direction Not applicable Sign of Sign of Pearson or Spearman correlation correlation (Spearman if not linear normal) Strength Size Cramer’s Size Spearman Size of Pearson Correlation V or lambda correlation (Spearman if not linear normal) Test Pearson, chi Test if Test if Pearson r = 0 square or Spearman rho (Test spearman r if non- Fisher’s exact =0 normal) Correlation • Assumes a linear association between variables • Pearson‟s correlation coefficient (known as r) is most commonly used correlation measure of linear relationships between 2 variables – (Spearman‟s rank correlation for non-linear ordered relationships) • Statistic measuring relationships between variables of interval (continuous) data, (e.g. census) • Census variables are interval data. the values are continuous, ranging from 0 - maximum • Generally put the „explanatory‟ (independent) variable as the x-axis • The variable you want to „explain‟ (dependent) is on the y- axis How to interpret Pearson’s correlation? measure is of how tightly the points cluster around an imaginary straight line through the scatterplot • r is ‘dimensionless’ number and can only be between 1 and -1 – an r of 1 = perfect positive relationship – an r of -1 = perfect negative relationship – an r of 0 = indicates no relationship Rule of thumb for interpreting the the magnitude of r Negative Description Positive Range Range 0.00 None 0.00 extent to which points -0.19 - -0.01 ‘Very weak’ 0.01 - 0.19 cluster tightly around the -0.39 - -0.20 ‘Weak’ 0.20 - 0.39 straight line -0.69 - -0.40 ‘Modest’ 0.40 – 0.69 -0.89 - -0.70 ‘Strong’ 0.70 – 0.89 -0.99 - -0.90 ‘Very strong’ 0.90 – 0.99 -1.00 Perfect 1.00 Significance testing • Can test to see whether the r is statistically significant • Key is the size of r and the size of sample • Seeking to reject the null hypothesis that the correlation coefficient is zero • Pearson‟s correlation coefficient can be tested only if both variables are normally distributed • (If not – test Spearman‟s correlation coefficient) • Look up r against a table of critical values for given degrees of freedom. If bigger, can reject H0 • Statistics package will report a p-value, a measure of significance • If p-value is less than 0.05 the correlation is significantly different to zero (with 95% certainty) • Can also use a t-statistic. again checking if critical value is exceeded Correlation limitations • With big sample sizes, almost everything is significantly related in purely statistical terms • Only works with linear relationships • Correlation is not causation. A high r may mean any one of these : – A causes B – some other factor causes A and B – B causes A – its just chance. another sample will be different • Need to use your knowledge, experience and common-sense as to likely underlying process. Is the relationship what you expect? Is it plausible? • Correlation is only concerned with the direction and strength of the relationship between values of two variables • Regression analysis determines the nature of that relationship and enables us to make predictions from it Statistics Sense • Danny Dorling‟s „Five Rules‟ • “If you have been concerned about your insecurities with statistics, don‟t be - you are normal - just try to use a few more simple facts to strengthen your arguments and try to feel less intimidated about the complex methods.” • 1. often there is little point in using statistics • 2. If you do use statistics make sure they can be understood • 3. do not overuse statistics in your work • 4. If you find a complex statistics useful, explain it clearly • 5. recognise and harness the power of statistics in geography • (Source: Chapter 21, “Using statistics to describe and explore data” in Clifford and Valentine (2003) Key Methods in Geography) Further reading • Danny Dorling‟s chapter in Clifford and Valentine (2003) Key Methods in Geography (chapter 21, “Using statistics to describe and explore data”) • 2 good stats books without equations! – Derek Rowntree (1981) Statistics without Tears: An Introduction for Non-Mathematicians (Science MATHEMATICS L5 ROW) – Michael Wood (2003) Making Sense of Statistics: A Non- Mathematical Approach (Main) • I strongly recommend David Ebdon, Statistics in Geography (Science GEOGRAPHY D 62 EBD) • Peter Rogerson (2001), Statistical Methods for Geography (Science GEOGRAPHY D 62 ROG) • Kenneth Berk and Patrick Carey, Data analysis with Microsoft Excel (Bartlett ARCHITECTURE BA 4.2 BER)

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posted: | 7/22/2011 |

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