HEALTH_Vol34No2_03

‡°√Á¥§«“¡√Ÿâ...∑“ß ∂‘µ‘

°“√µ√«® Õ∫°“√·®°·®ß¢ÕߢâÕ¡Ÿ≈

燰√Á¥§«“¡√Ÿâ...∑“ß ∂‘µ‘é ©∫—∫π’È §≥–∫√√≥“∏‘°“√«“√ “√°“√ à߇ √‘¡ ÿ¢¿“æ·≈–Õπ“¡—‘ßË ·«¥≈âÕ¡‰¥âπ”‡√◊ÕË ß °“√µ√«® Õ∫°“√·®°·®ß¢ÕߢâÕ¡Ÿ≈ ®“°π“ß “«æ‘¡æå√¿— ‡µÁßµ√–°Ÿ≈‡®√‘≠ ·Àà ß Àπà « ¬√–∫“¥«‘ ∑ ¬“§≈‘ π‘ ° ∂“π à ß ‡ √‘ ¡ °“√«‘ ®— ¬ §≥–·æ∑¬»“ µ√å »‘ √‘ √ “™æ¬“∫“≈ ¡À“«‘∑¬“≈—¬¡À‘¥≈ ¡“·≈°‡ª≈’ˬπ‡√’¬π√Ÿâ°—∫∑à“π ¡“™‘°«“√ “√œ ´÷Ë߇ÀÁπ«à“¬—ß¡’π—°«‘®—¬À√◊Õπ—° «‘™“°“√∫“ß∑à“π∑’Ë¡—°‡≈◊Õ°π”‡ πÕ§à“ ∂‘µ‘æ√√≥π“À√◊Õ°“√‡≈◊Õ°„™â ∂‘µ‘∑¥ Õ∫„π°“√∑”°“√ »÷°…“«‘®—¬‚¥¬¡‘‰¥â∑”°“√µ√«® Õ∫°“√·®°·®ß¢ÕߢâÕ¡Ÿ≈‡∫◊ÈÕßµâπ‡æ◊ËÕ§«“¡∂Ÿ°µâÕß àߺ≈„À⇰‘¥ §«“¡‰¡à‡À¡“– ¡„π¢âÕ √ÿª√“¬ß“πº≈°“√»÷°…“ ¥—ßπ—È𠧫“¡√Ÿâ‡°’ˬ«°—∫°“√µ√«® Õ∫°“√·®° ·®ß¢ÕߢâÕ¡Ÿ≈µàÕ‰ªπ’È®–‡ªìπª√–‚¬™πå·°à∑à“π ¡“™‘°«“√ “√œ ‡æ◊Ëՙ૬°—π √â“ß¡“µ√∞“πß“π «‘®—¬„Àâ§ß§ÿ≥¿“欑Ëߢ÷Èπ ·≈–‡™àπ‡§¬ À“°∑à“π ¡“™‘°«“√ “√œ π—°«‘™“°“√ √«¡∑—Èß∑à“πºŸâ π„® ¡’ §«“¡ª√– ߧ宖√à«¡ àß·ß৑¥ ·≈–¡ÿ¡¡Õߧ«“¡√Ÿâ∑“ß ∂‘µ‘‡æ◊ËÕ‡ªìπ°“√‡√’¬π√Ÿâ√à«¡°—π §≥– ∫√√≥“∏‘°“√œ ¬‘π¥’µâÕπ√—∫ Ÿàæ◊Èπ∑’Ë·Ààß°“√‡√’¬π√Ÿâ ≥ §Õ≈—¡πåπ’È...

°“√µ√«® Õ∫°“√·®°·®ß¢ÕߢâÕ¡Ÿ≈ æ‘¡æå√¿— ‡µÁßµ√–°Ÿ≈‡®√‘≠*

°

“√𔇠πÕ§à“ ∂‘µ‘æ√√≥π“ ”À√—∫ ¢âÕ¡Ÿ≈µàÕ‡π◊ËÕß ‡™àπ Õ“¬ÿ √–¥—∫ πÈ”µ“≈„π‡≈◊Õ¥ πÈ”Àπ—° à«π Ÿß √–¥—∫§«“¡¥—π ‚≈À‘µ ‡ªìπµâπ à«π„À≠à𬑠¡π”‡ πÕ§à“°≈“ß ·≈– §à“°“√°√–®“¬ ‰¥â·°à §à“‡©≈’ˬ ·≈– à«π‡∫’Ë¬ß ‡∫π¡“µ√∞“𠵓¡≈”¥—∫ ‚¥¬¡’‡ß◊ËÕπ‰¢«à“°“√ 𔇠πÕ§à“ ∂‘µ‘æ√√≥π“®–‡À¡“– ¡‡¡◊ËÕ¢âÕ¡Ÿ≈ ‡À≈à“π—Èπ¡’°“√·®°·®ßª°µ‘ À√◊Õ·¡â·µà„π°“√ ∑¥ Õ∫ ¡¡µ‘∞“π∑“ß ∂‘µ‘ ‚¥¬°“√„™â ∂‘µ‘ t-test, °“√«‘‡§√“–À姫“¡·ª√ª√«π(One-way Analysis of Variance) À√◊Õ ∂‘µ‘Õ‘ßæ“√“¡‘‡µÕ√åÕ◊ËπÊ °Á®–¡’¢âÕµ°≈߇∫◊ÈÕßµâπ¢Õß°“√„™â ∂‘µ‘π—Èπ ´÷Ëß¡—° ®–‡°’ˬ«¢âÕß°—∫°“√·®°·®ß¢ÕߢâÕ¡Ÿ≈‡ ¡Õ ·≈– ®–„™â ∂‘µ‘Õ‘ßæ“√“¡‘‡µÕ√å ‰¥â°ÁµàÕ‡¡◊ËÕ¢âÕ¡Ÿ≈(µ—«

·ª√º≈≈—æ∏å) ¡’°“√·®°·®ßª°µ‘ ®÷ß®–∑”„Àâº≈ °“√∑¥ Õ∫ ¡¡µ‘∞“π‰¥â¢âÕ √ÿª∑’Ë∂Ÿ°µâÕß ‡æ√“–©–π—Èπ ‰¡à«à“®–‡ªìπ°“√𔇠πÕ§à“ ∂‘µ‘æ√√≥π“ À√◊Õ·¡â·µà°“√∑¥ Õ∫ ¡¡µ‘∞“π ®÷ߧ«√¡’°“√µ√«® Õ∫°“√·®°·®ß¢Õßµ—«·ª√ ‡À≈à“π—Èπ°àÕπ«à“¡’°“√·®°·®ß·∫∫ª°µ‘À√◊Õ‰¡à ‡æ◊ËÕ„™â ”À√—∫°“√µ—¥ ‘π„®„π°“√‡≈◊Õ°π”‡ πÕ §à“ ∂‘µ‘ À√◊Õ°“√‡≈◊Õ°„™â ∂‘µ‘ ”À√—∫°“√∑¥ Õ∫ ∑’ˇÀ¡“– ¡ ‚¥¬°“√µ√«® Õ∫°“√·®°·®ß¢Õß µ—«·ª√«à“¡’°“√·®°·®ßª°µ‘À√◊Õ‰¡à “¡“√∂ µ√«® Õ∫‰¥â 2 «‘∏’ ¥—ßπ’È 1. °“√æ‘®“√≥“‡∫◊ÈÕßµâπ (Informal tests) - °“√æ‘ ® “√≥“®“°§à “ ∂‘ µ‘ ‡ ™‘ ß

*Àπ૬√–∫“¥«‘∑¬“§≈‘π‘° ∂“π à߇ √‘¡°“√«‘®—¬ §≥–·æ∑¬»“ µ√廑√‘√“™æ¬“∫“≈ ¡À“«‘∑¬“≈—¬¡À‘¥≈ THAILAND JOURNAL OF HEALTH PROMOTION AND ENVIRONMENTAL HEALTH A p r i l - J u n e 2 0 1 1

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‡°√Á¥§«“¡√Ÿâ...∑“ß ∂‘µ‘ æ√√≥π“ °“√æ‘®“√≥“®“°§à“ ∂‘µ‘‡™‘ßæ√√≥π“ “¡“√∂æ‘®“√≥“‰¥â®“°§à“‡©≈’ˬ à«π‡∫’ˬ߇∫π ¡“µ√∞“π(SD) ·≈–®“°§à“¡—∏¬∞“π ‚¥¬∑’ËÀ“° æ∫«à“§à“ SD ¡’§à“¡“°°«à“§√÷ËßÀπ÷ËߢÕß§à“‡©≈’ËÀ√◊ Õ À“°æ∫«à “ §à “ ‡©≈’Ë ¬ ¡’ §à “ Ÿ ß À√◊ Õ µË” °«à “ §à “ ¡—∏¬∞“π Õ“®®–· ¥ß‰¥â«à“¢âÕ¡Ÿ≈¡’≈—°…≥–°“√ ·®°·®ß·∫∫‰¡àª°µ‘ °“√æ‘®“√≥“®“°§à“§«“¡‚¥àß (Kurtosis) ·≈–§à“§«“¡‡∫â (Skewness) ¥—ßπ’È - §à“§«“¡‚¥àß (Kurtosis) · ¥ß „Àâ‡ÀÁπ∂÷ߧ«“¡ Ÿß¢Õ߇ âπ‚§âß®“°°“√·®°·®ß ¢ÕߢâÕ¡Ÿ≈ ∂â“§à“§«“¡‚¥àß¡’§à“‡∑à“ 3 · ¥ß«à“ °“√·®°·®ß¢ÕߢâÕ¡Ÿ≈¡’§«“¡‚¥àßæÕ‡À¡“–µ“¡ ≈—°…≥–¢Õß°“√·®°·®ß·∫∫ª°µ‘ ∂â“À“°§à“ §«“¡‚¥àß¡’§à“πâÕ¬°«à“ 3 · ¥ß«à“°“√·®°·®ß ¢ÕߢâÕ¡Ÿ≈¡’≈—°…≥–·∫πÀ√◊Õ‡µ’Ȭ ∂â“§à“§«“¡ ‚¥àß¡“°°«à“ 3 · ¥ß«à“¢âÕ¡Ÿ≈¡’≈—°…≥–‚¥àßÀ√◊Õ Ÿß·À≈¡ ´÷ËßÀ“°¢âÕ¡Ÿ≈®–¡’≈—°…≥–°“√·®°·®ß ·∫∫ª°µ‘§«√®–¡’§à“§«“¡‚¥à߇¢â“„°≈â 3 - §à“§«“¡‡∫â (Skewness) ‡ªìπ§à“ ∑’Ë · ¥ß„Àâ ‡ ÀÁ π ∂÷ ß §«“¡º‘ ¥ ª°µ‘ ¢ Õߢâ Õ ¡Ÿ ≈ ∂â “ ‡ â 𠂧â ß §«“¡∂’Ë ‡ ∫â ‰ ª∑“ß´â “ ¬À√◊ Õ ‡∫â ≈ ∫ (skewed to the left À√◊Õ negative skew) ·≈â« Mean<Median <Mode · ¥ß«à“¢âÕ¡Ÿ≈ à«π „À≠à ¡’ §à “ ¡“°(Õ“®‡ªì π §à “ º‘ ¥ ª°µ‘ ∑’Ë µâ Õ ßÀ“ “‡Àµÿ·≈–À“∑“ß·°â ‰¢‰¥â) ∂Ⓡ âπ‚§âߧ«“¡∂’Ë ‰ª∑“ߢ«“À√◊Õ‡∫⢫“ (skewed to the right À√◊Õ positive skew) ·≈– Mode<Median<Mean · ¥ß«à“¢âÕ¡Ÿ≈ à«π„À≠à¡’§à“πâÕ°“√æ‘ ® “√≥“§«“¡‡∫â „ Àâ æ‘ ® “ √ ≥ “ ‡§√◊ËÕßÀ¡“¬ ¥—ßπ’È Skewness = - · ¥ß«à“ ‡∫â´â“Skewness = 0 · ¥ß«à“ ‰¡à¡’§«“¡‡∫â 32

‡¡…“¬π - ¡‘∂ÿπ“¬π 2554

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Skewness = + · ¥ß«à“ ‡∫⢫“ - °“√æ‘®“√≥“®“°°√“ø °“√µ√«® Õ∫¥Ÿ ° “√·®°·®ß¢Õß ¢â Õ ¡Ÿ ≈ ®“°°√“ø “¡“√∂∑”‰¥â À ≈“¬«‘ ∏’ ‡ ™à 𠇥’¬«°—𠇙àπ °√“øŒ‘ ‚µ·°√¡(Histogram), Box-plot, Stem-and-leaf, Normal probability plot, Q-Q plot ‡ªìπµâπ À“°æ‘®“√≥“ ®“°°√“ø·≈â«æ∫«à“¡’≈—°…≥–°“√·®°·®ß¢Õß ¢âÕ¡Ÿ≈∑’ˉ¡à ¡¡“µ√ °Á· ¥ß«à“¢âÕ¡Ÿ≈Õ“®®–¡’°“√ ·®°·®ß·∫∫‰¡àª°µ‘‰¥â ¥—ß¿“æ∑’Ë 1 2. °“√∑¥ Õ∫∑“ß ∂‘ µ‘ (Formal tests) „π°“√µ√«® Õ∫°“√·®°·®ß¢ÕߢâÕ¡Ÿ≈ ®“°°√“ø À√◊Õ§à“ ∂‘µ‘‡™‘ßæ√√≥π“ ‡ªìπ‡æ’¬ß °“√µ√«® Õ∫‡∫◊ÈÕßµâπ∑’Ë∑”„Àâ∑√“∫≈—°…≥–°“√ ·®°·®ß¢Õߢâ Õ ¡Ÿ ≈ ‡∑à “ π—È π À“°®–µâ Õ ß √ÿ ª «à“¢âÕ¡Ÿ≈¡’°“√·®°·®ß·∫∫ª°µ‘À√◊Õ‰¡à ®–µâÕß ∑”°“√∑¥ Õ∫∑“ß ∂‘ µ‘ ´÷Ë ß ∂‘ µ‘ ∑’Ë „ ™â „ π°“√ ∑¥ Õ∫°“√·®°·®ß¢ÕߢâÕ¡Ÿ≈∑’πË ¬‘ ¡„™â‚¥¬∑—«Ë ‰ª ‰¥â·°à Kolmogorov-Smirnov test(Lilliefors significance correction) ®–„™â∑¥ Õ∫°“√ ·®°·®ß¢ÕߢâÕ¡Ÿ≈°√≥’∑µ’Ë «— Õ¬à“ß¡’¢π“¥„À≠à(n> 50) ·≈– Shapiro-Wilk test ®–„™â∑¥ Õ∫°“√ ·®°·®ß¢Õߢâ Õ ¡Ÿ ≈ °√≥’ ∑’Ë µ— « Õ¬à “ ß¡’ ¢ π“¥ ‡≈Á°(n≤50)∑—Èßπ’Ⱥ≈°“√∑¥ Õ∫¢Õß∑—Èß Õßµ—« Õ“®®–‰¡àµ√ß°—π‰¥â ∑—ßÈ π’‡È π◊ÕË ß®“° Shapiro-Wilk test ¡’§«“¡‰«µàÕ§à“¢âÕ¡Ÿ≈∑’Ë¡’§«“¡º‘¥ª°µ‘¡“° ®÷ß∑”„Àâ‚Õ°“ ∑’˺≈°“√∑¥ Õ∫®–æ∫«à“¢âÕ¡Ÿ≈¡’ °“√·®°·®ß·∫∫‰¡àª°µ‘‰¥â ‚¥¬ºŸâ«‘®—¬§«√®– æ‘®“√≥“°“√·®°·®ß¢ÕߢâÕ¡Ÿ≈®“°§à“ ∂‘µ‘‡™‘ß æ√√≥π“·≈–°√“ø√à«¡¥â«¬

¢—ÈπµÕπ°“√∑¥ Õ∫ ¡¡µ‘∞“π °“√·®°·®ß

°“√µ√«® Õ∫°“√·®°·®ß¢ÕߢâÕ¡Ÿ≈

¿“æ∑’Ë 1 °“√·®°·®ß·∫∫‡∫⢫“ ·∫∫ª°µ‘ ·≈–·∫∫‡∫â´â“‡∫⢫“ (skewed to the right À√◊Õ positive skew)

ª°µ‘ (normal)

‡∫â´â“(skewed to the left À√◊Õ negative skew)

°“√∑¥ Õ∫°“√·®°·®ß®–¡’¢—ÈπµÕπ„π °“√∑¥ Õ∫‡™àπ‡¥’¬«°—π°—∫°“√∑¥ Õ∫ ¡¡µ‘∞“π ¥—ßπ’È 1. µ—Èß ¡¡µ‘∞“π ¡ µ‘∞“πÀ≈—°(H0) : √–¥—∫πÈ”µ“≈ „π‡≈◊Õ¥ ¡’°“√·®°·®ß·∫∫ª°µ‘ ¡¡µ‘ ∞ “π∑“߇≈◊ Õ °(HA) : √–¥—∫ πÈ”µ“≈„π‡≈◊Õ¥ ‰¡à¡°’ “√·®°·®ß·∫∫ª°µ‘

2. ‡≈◊Õ° ∂‘µ‘∑’Ë„™â „π°“√∑¥ Õ∫ ‡≈◊Õ°„™â ∂‘µ‘ Kolmogorov-Smirnov test À√◊Õ Shapiro-Wilk test 3. °”Àπ¥√–¥—∫π—¬ ”§—≠∑“ß ∂‘µ∑‘ ’Ë 0.05 4. §”π«≥§à“ ∂‘µ‘·≈– p-value 5. µ—¥ ‘π„®·≈– √ÿªº≈ p-value ≤α · ¥ß«à“ µ—«·ª√ x ‰¡à¡°’ “√·®°·®ß·∫∫ª°µ‘

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‡°√Á¥§«“¡√Ÿâ...∑“ß ∂‘µ‘ p-value >α · ¥ß«à“ µ—«·ª√ x ¡’°“√·®°·®ß·∫∫ª°µ‘

”À√—∫°“√∑¥ Õ∫°“√·®°·®ß¢ÕߢâÕ¡Ÿ≈·≈–°¥

§” —Ëß„π‚ª√·°√¡SPSS 1. ‡ªî¥‚ª√·°√¡ SPSS ·≈–‰ø≈å¢âÕ¡Ÿ≈ „π‡¡πŸ ∫ “√å ‡ ≈◊ Õ ° Analyze➱Descriptive Statistics➱Explore...

2. Àπâ “ µà “ ߧ” —Ë ß Explore „Àâ ‡ ≈◊ Õ ° µ—«·ª√µàÕ‡π◊ËÕß∑’˵âÕß°“√∑¥ Õ∫°“√·®°·®ß „ à≈߉ª„π™àÕß Dependent List (µ—«·ª√µàÕ‡π◊ÕË ß ‡™àπ Õ“¬ÿ πÈ”Àπ—° ‡ªìπµâπ) 1 ´÷Ëß„π∑’Ëπ’ȵ—«·ª√∑’Ë µâÕß°“√∑¥ Õ∫§◊Õ√–¥—∫πÈ”µ“≈„π‡≈◊Õ¥ (FBS)

Continue 5 ➱OK 6 5. º≈≈—æ∏å¢Õß‚ª√·°√¡ SPSS ®“°º≈≈——æ∏å¢Õß‚ª√·°√¡ SPSS æ∫«à“ √ – ¥— ∫ πÈ” µ “ ≈ „ π ‡ ≈◊ Õ ¥ ¡’ §à “ ‡ © ≈’Ë ¬ ‡ ∑à “ °— ∫ 137.4(SD=9.0) ·≈–¡’§à“§«“¡‚¥àß ·≈–§«“¡ ‡∫â‡∑à“°—∫ -0.488 ·≈– 0.089 µ“¡≈”¥—∫ · ¥ß „Àâ ‡ ÀÁ π «à “ √–¥— ∫ πÈ” µ“≈„π‡≈◊ Õ ¥πà “ ®–¡’ ° “√ ·®°·®ßª°µ‘ ´÷Ëß Õ¥§≈âÕß°—∫º≈°“√∑¥ Õ∫ ¡¡µ‘∞“π®“°µ“√“ß Test of Normality ‚¥º≈¢Õß Kolmogorov-Smirnov ¡’ §à “ pvalue=0.200 ·≈–®“° Shapiro-Wilk ¡’§à“ pvalue=0.129 ´÷Ëß¡“°°«à“§à“√–¥—∫π—¬ ”§—≠∑“ß ∂‘µ‘ 0.05 ¥—ßπ—Èπ ®÷ß √ÿª‰¥â«à“√–¥—∫πÈ”µ“≈„π ‡≈◊Õ¥¡’°“√·®°·®ß·∫∫ª°µ‘ (¥—ßµ“√“ß)

√ÿª 3. „π™àÕß Display ‡≈◊Õ° Both 2 ·≈– „π‡¡πŸ„Àâ‡≈◊Õ° Plots... 3 4. „π‡¡πŸ Plots ®–ª√“°ØÀπ⓵à“ߥ—ß √Ÿª„Àâ‡≈◊Õ° Normality Plots with tests 4 34

‡¡…“¬π - ¡‘∂ÿπ“¬π 2554

°√¡Õπ“¡—¬ à߇ √‘¡„Àâ§π‰∑¬ ÿ¢¿“楒

‡æ◊ËÕ§«“¡∂Ÿ°µâÕߢÕߺ≈°“√»÷°…“®“° °“√‡≈◊Õ°„™â ∂‘µ‘æ√√≥π“„π°“√𔇠πÕ¢âÕ¡Ÿ≈ ·≈–°“√ √ÿ ª º≈°“√»÷ ° …“®“°°“√„™â ∂‘ µ‘ ∑ ¥ Õ∫‰¥â‡À¡“– ¡ ºŸâ«‘®—¬§«√¡’°“√µ√«® Õ∫°“√ ·®°·®ß¢ÕߢâÕ¡Ÿ≈‡∫◊ÈÕßµâπ°àÕπ∑ÿ°§√—Èß ∑—Èßπ’ÈÕ“® æ‘®“√≥“®“°°√“ø À√◊Õ°“√∑¥ Õ∫∑“ß ∂‘µ‘

°“√µ√«® Õ∫°“√·®°·®ß¢ÕߢâÕ¡Ÿ≈

Descriptives FBS(mg%)

Statistic 137.4022 135.0703 139.7341 137.3339 137.7900 81.485 9.02692 120.64 158.59 37.95 14.21 .089 -.488

Mean 95% Confidence Interval Lower Bound for Mean Upper Bound 5% Trimmed Mean Median Variance Std. Deviation Minimum Maximum Range Interquartile Range Skewness Kurtosis

Std.Error 1.16537

.309 .608

Tests of Normality Kolmogorov-Smirnova Statistic df Sig. FBS

.093

60

Shapiro-Wilk Statistic df

.200*

.969

60

Sig. .129

a.Lilliefors Significance Correction *.This is a lower bound of the true significance. ‡°’ˬ«°—∫°“√·®°·®ßª°µ‘¢ÕߢâÕ¡Ÿ≈ À“°æ∫ «à“¢âÕ¡Ÿ≈‰¡à¡’°“√·®°·®ßª°µ‘ ºŸâ«‘®—¬§«√®– ‡≈◊Õ°„™â ∂‘µ‘æ√√≥π“ ·≈– ∂‘µ‘∑¥ Õ∫„Àâ‡À¡“– ¡ ∂â “ ºŸâ «‘ ®— ¬ µâ Õ ß°“√„™â ∂‘ µ‘ ∑ ¥ Õ∫·∫∫Õ‘ ß æ“√“¡‘‡µÕ√å·µà¢Õâ ¡Ÿ≈‰¡à¡°’ “√·®°·®ßª°µ‘ ºŸ«â ®‘ ¬— §«√®–∑”°“√·ª≈ߢâÕ¡Ÿ≈(Transformation)

æ√âÕ¡∑—ßÈ ∑¥ Õ∫°“√·®°·®ß¢ÕߢâÕ¡Ÿ≈∑’‰Ë ¥â∑”°“√ ·ª≈ß·≈â « «à “ ¡’ ° “√·®°·®ßª°µ‘ À √◊ Õ ‰¡à À“° ·ª≈ߢâÕ¡Ÿ≈·≈â«·µà¢âÕ¡Ÿ≈¬—ߧ߉¡à¡’°“√·®°·®ß ª°µ‘ ºŸâ«‘®—¬§«√„™â ∂‘µ‘∑¥ Õ∫·∫∫‰¡àÕ‘ßæ“√“¡‘ ‡µÕ√å

‡Õ° “√Õâ“ßÕ‘ß 1.Armitage P., Berry G., and Matthews J.N.S., Statistical method in medical research.4th ed, Oxford: Blackwell; 2002.

2. Beth Dawson-Saunders, Trapp RG. Basic and clinical biostatistics. Prentice Hell International Inc;1990 3.Chan YH. Basic statistics for doctors-Biostatistics 101:Data Presentation. Singapore Med J 2003;44(6):280-5 4.Zar JH. Biostatistical analysis. 4th ed, New Jersey:Prentice-Hall;1999.

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